One of the biggest reasons people take that long-haul flight to Thailand year after year is the water. Not just the temperature of it — a year-round average of around 28°C (82°F) — but what lives beneath the surface of it. Thailand’s two coastlines sit in fundamentally different ocean systems: the Andaman Sea to the west, and the Gulf of Thailand to the east. Between them, they hold over 900 species of reef fish, nearly 500 islands, and some of the most celebrated dive sites in Asia. Whether you have never worn a mask before or you are working towards your Divemaster, Thailand has a dive for you — and this guide covers everything from where to go and what it will cost, to a few things most guides skip entirely.
Understanding Thailand’s Two Coasts
Before booking anything, it is worth knowing which coast you are dealing with and why it matters. The Andaman Sea (west coast) is home to the headline dive sites — the Similan Islands, Richelieu Rock, Koh Phi Phi, and Phuket. The water here is generally clearer, the marine megafauna more abundant, and the dive infrastructure more developed. The Gulf of Thailand (east coast) includes Koh Samui, Koh Tao, and Sail Rock. It is slightly warmer, conditions are different by season, and it is where most beginner courses happen — particularly on Koh Tao, which has a global reputation as the world’s most affordable place to get certified.
The two coasts are also separated by the Malay Peninsula, which means weather patterns are different and almost opposite. Understanding this is what unlocks the seasonality question that confuses so many first-time visitors.
Best Time to Dive in Thailand
The honest answer is: it depends entirely on which coast you are visiting.
- Andaman Sea (Phuket, Khao Lak, Koh Lanta, Similan Islands, Surin Islands, Koh Phi Phi): The season runs from November to April. Outside of these months, the southwest monsoon brings swells, reduced visibility, and most liveaboard operators close down entirely. The absolute peak window — especially for large pelagic sightings including whale sharks and manta rays — is February to April.
- Gulf of Thailand (Koh Tao, Koh Samui, Koh Pha Ngan, Sail Rock): Diving here is best from May to September, when the Andaman side is at its worst. The Gulf Coast gets its own shorter storm period from October to December, but conditions are generally more forgiving year-round than the Andaman side.
One site worth flagging individually: Koh Tao offers some of its best diving from June to September, when visibility is excellent and the island is slightly less crowded than its peak party season. This is a detail frequently omitted from mainstream guides.
Your Duty: Reef Ethics and the Sunscreen Law
Thailand’s reefs have had a difficult few years. In mid-2024, a global mass bleaching event — the fourth on record, declared by NOAA and the International Coral Reef Initiative — affected between 60% and 80% of coral in both the Andaman Sea and the Gulf of Thailand. A 2025 report from Thailand’s Department of Marine and Coastal Resources confirmed that roughly 60% of bleached corals had recovered, but around 40% were lost. A coast-to-coast study published in early 2026 by researchers at the Aow Thai Marine Ecology Center, based on surveys conducted between 2022 and 2024, found that many of Thailand’s reefs are now structurally simpler ecosystems than in previous decades — with complex branching coral species like Acropora less prevalent than they once were, and the fire corals (Millepora) that once lined the Gulf of Thailand now largely absent from their former sites.
This is not a reason to skip diving in Thailand. It is a reason to be a more informed and responsible diver when you are here.
The single most practical thing you can do before you enter the water is to check your sunscreen. In August 2021, Thailand’s Department of National Parks, Wildlife and Plant Conservation introduced a nationwide ban on sunscreens containing any of four specific chemicals inside all marine national parks. The four prohibited substances are oxybenzone, octinoxate, 4-methylbenzylidene camphor (4-MBC), and butylparaben. The Tourism Authority of Thailand confirmed that violators face fines of up to 100,000 Thai Baht (approximately US$2,700). The ban covers the Similan Islands, Surin Islands, Koh Phi Phi, Mu Ko Ang Thong, and every other national marine park. Enforcement has actively strengthened since 2025.
To check your sunscreen, look at the active ingredients list on the back of the bottle. If you see any of those four names, leave the bottle behind. Buy reef-safe mineral sunscreen (zinc oxide or titanium dioxide-based) before you travel, or pick it up at Boots or Watsons pharmacies in any Thai airport or mall for around 300–800 Baht. A rash guard or UV-protection wetsuit is an even better option — it eliminates the need for sunscreen on your body entirely while in the water, and most reputable dive centres sell or rent them.
Beyond sunscreen: never touch coral, maintain good buoyancy before descending on a live reef, do not feed fish, and do not buy souvenirs made from shells or coral. These are basic courtesies, but they carry real ecological weight in a reef system that is currently under stress.
Understanding Certification Levels
If you are planning to scuba dive in Thailand rather than just snorkel, it helps to understand where you sit in the certification ladder before you start pricing up trips, because your certification level directly determines which sites you can access.
- No certification / Discover Scuba Diving (DSD): A one-day introductory experience conducted in confined water and a shallow ocean dive. You are supervised one-to-one throughout. Great for deciding whether you want to pursue full certification. Costs around 2,500–3,500 Baht on Koh Tao.
- PADI Open Water Diver: The world’s most popular entry-level scuba qualification. Certifies you to dive to 18 metres (59 feet) worldwide with a qualified buddy. Takes 3–4 days. This is the standard course recommended for most first-time divers.
- PADI Advanced Open Water Diver: Adds five speciality adventure dives — typically deep diving (to 30 metres / 98 feet), navigation, night diving, and two electives. Unlocks sites like the King Cruiser Wreck and Chumphon Pinnacle that have a depth requirement. Takes 2 days.
- Rescue Diver and beyond: For divers progressing toward Divemaster. Adds emergency response skills and significantly increases your safety awareness in the water.
Both PADI and NAUI (National Association of Underwater Instructors) are globally respected. SSI (Scuba Schools International) is equally valid and is widely available on Koh Tao. All three agencies are aligned to the World Recreational Scuba Training Council (WRSTC), meaning certifications are interchangeable.
Getting Your Diving Kit
Unless you dive obsessively and travel with checked luggage to match, there is no need to bring your own gear. The vast majority of dive centres provide a full equipment package — wetsuit, BCD, regulator, tank, mask, fins, and computer — either included in the course price or as an add-on rental.
- Mask and snorkel rental: Around 50–100 Baht, with a 200 Baht refundable deposit on return
- Full snorkel kit (mask, fins, snorkel): Around 250 Baht for the day
- Full scuba gear rental (wetsuit, BCD, regulator, tank): Around 600 Baht per dive, excluding deposit
- Underwater camera rental: 1,000–1,500 Baht per day; most centres offer GoPro-style units. Worth it for a liveaboard trip to Richelieu Rock
Always ask whether gear rental is included in your course or day-trip price before booking. This single question saves a lot of people a lot of money on arrival. Also ask whether a dive computer is included — on deep sites like the King Cruiser Wreck, having your own NDL (no-decompression limit) readout is important, not optional.
Diving Courses and Prices on Koh Tao
Koh Tao is frequently cited as the most popular place in the world to get PADI-certified, and it is consistently among the most affordable. An Open Water course on Koh Tao runs approximately 10,000–12,950 Baht (roughly £200–250 / US$270–350), covering all instruction, dives, gear, and your certification card. For context: the same course in Western Europe or North America typically costs three to four times as much. The island has over 50 dive schools competing for business, which keeps standards high and prices competitive. A minimum-price agreement has been in effect for several years, meaning very cheap off-the-street offers should be treated with caution — they usually involve larger student-to-instructor ratios or gear in poorer condition.
- Discover Scuba Diving (1 day): 2,500–3,500 Baht
- Open Water certification (3–4 days): 10,000–12,950 Baht
- Advanced Open Water (2 days, 5 dives): 8,500–11,900 Baht
- Rescue Diver: 10,000–12,000 Baht
- Fun dives (already certified): from 800 Baht per dive
Questions to Ask a Diving Centre Before You Book
Are you insured — and are you a member of PADI or a recognised agency?
PADI-affiliated dive centres in Thailand are held to a strict code of practice covering instructor-to-student ratios, equipment standards, and emergency procedures. NAUI-affiliated centres operate to the same standard. You can search both organisations’ websites to check affiliations and read independent reviews. If a centre is not affiliated with any recognised training agency, that is a significant red flag.
Personal diving insurance is separate from the centre’s liability cover. It costs under 50 Baht per day and is worth every satang. Dive Alert Network (DAN) is the best known provider and offers short-term policies suitable for holiday diving.
What is the student-to-instructor ratio?
PADI standards cap the Open Water course at a 4:1 student-to-instructor ratio for ocean dives. Some centres on busy days push this. Smaller ratios mean more individual attention — especially important on your first open-water dives where buoyancy and ear equalisation are the two things most beginners struggle with.
How big is the boat, and what emergency equipment is on board?
Ask to see photos of the boat before committing. Ask whether it carries oxygen, a first-aid kit, and a functioning marine radio. These are not paranoid questions — they are the basic standard for any legitimate operator. Emergency oxygen is particularly important: it is the first-line treatment for decompression sickness symptoms while waiting for evacuation, and not every budget operator carries it.
What is included in the price?
Confirm whether the gear rental, national park fees (these apply to the Similan and Surin Islands), fuel levies, and any marine reserve entry fees are included. National park entry fees on the Similan Islands are currently around 700 Baht for foreigners per day and are not always bundled into liveaboard or day-trip prices.
Snorkelling in Thailand: A Dedicated Section for Non-Divers
Snorkelling is not the consolation prize for people who do not dive — in the right spots, it is the better option. Thailand’s shallow reef systems, particularly around Koh Tao, the Surin Islands, and the shallower bays of the Phi Phi archipelago, are accessible in as little as 1–3 metres (3–10 feet) of water. You do not need a certification, training dives, or expensive gear to see green sea turtles, blacktip reef sharks, clownfish, and live coral gardens.
The key for snorkellers is to choose locations with calm conditions and minimal boat traffic in the area you are snorkelling. Organised snorkel tours from reputable operators are strongly recommended over hiring a longtail and going alone — guides know where the marine life actually is on any given day, and they can keep you away from boat lanes. Most multi-island snorkel day trips in the Gulf depart from Koh Tao or Koh Samui; on the Andaman side, tours from Ao Nang, Krabi, and Phuket cover the Phi Phi Islands and, less commonly, the outer Surin Islands.
The one piece of gear worth buying rather than renting is a good quality mask with a silicone skirt that fits your face. A cheap mask that leaks turns a great snorkel into a frustrating one. Bring your own if you can — they cost from 300 Baht at dive shops across Thailand.
Going Diving: Your Main Options
- Day trips: The standard format. Depart from a nearby town or pier in the morning, complete 2–3 dives at nearby sites, return by late afternoon. Best for those based in Phuket, Krabi, Koh Tao, or Koh Samui.
- Night dives: Most dive centres offer these as a supplement to day dives. The reef at night is a completely different environment — octopus, crabs, sleeping reef fish, and the bioluminescence that occurs when you move through the water.
- Liveaboards: 2–7 day diving expeditions on a purpose-built boat, departing from Khao Lak, Phuket, or Ranong. These are the only practical way to reach the Similan Islands, Surin Islands, and Richelieu Rock for multiple dives across multiple days. A 4–5 day Similan and Surin liveaboard with 15–20+ dives costs from around 18,000–30,000 Baht depending on the season and operator. This is the format serious divers plan their entire Thailand trip around.
Top Places to Snorkel and Dive in Thailand
Koh Tao
Koh Tao — whose name literally means Turtle Island — is the undisputed centre of Thailand’s dive training industry. More PADI Open Water certificates are issued here than at almost any other single location on earth. For beginners, the island’s sheltered bays offer calm, clear conditions in water that rarely drops below 28°C (82°F). Shark Bay in the south is the go-to spot for seeing green sea turtles from the beach — some individuals have been returning to the same patch of seagrass for years and are well accustomed to divers and snorkellers.
For experienced divers, Chumphon Pinnacle — located about 12 miles (19 km) northwest of the island — is the standout site. Four granite pinnacles rise from 40 metres (131 feet) to about 14 metres (46 feet) below the surface, and whale shark sightings are recorded here more frequently than at most other Gulf sites. Sail Rock, roughly 14 miles (22 km) northeast of Koh Tao, is widely considered the finest dive site in the Gulf of Thailand: a single exposed rock with a dramatic chimney passage running vertically through the interior that divers can ascend from inside. Barracuda, giant trevally, and yellowfin tuna are regular visitors.
Richelieu Rock
Most Thai dive guides place Richelieu Rock at the top of the list — and with justification. This isolated horseshoe-shaped pinnacle sits approximately 124 miles (200 km) northwest of Phuket, part of the Mu Ko Surin Marine National Park and accessible only by liveaboard. The name comes from Jacques Cousteau, who reportedly named the site after the purple soft corals draped across it that reminded him of Cardinal Richelieu’s robes. Cousteau is said to have called it one of the world’s finest dive sites — though various versions of this claim circulate and should be treated as lore rather than documented fact.
What is documented is the marine life. The site’s position creates nutrient-rich upwellings that attract plankton blooms, which in turn attract pelagics. Whale shark encounters are more reliably reported here than anywhere else in Thailand — February to April is the peak window, though sightings can happen anywhere between November and April. Manta rays are regular visitors, particularly at nearby Koh Bon. The site also rewards macro photographers: ornate ghost pipefish, harlequin shrimp, seahorses, and nudibranchs are found in the overhangs and ledges on most dives. Visibility commonly reaches 20–40 metres (66–131 feet), though high plankton density during whale shark season can reduce this — which, counterintuitively, is the sign you want to see. Depth runs from 5 metres (16 feet) on the shallowest sections to around 30 metres (98 feet). Mild to moderate currents make it suitable for intermediate divers and above.
Similan Islands
The Similan Islands National Marine Park — a chain of nine granite islands about 60 miles (97 km) northwest of Phuket — is the most celebrated dive destination on the Andaman side for good reason. The underwater topography switches between dramatic boulder formations on the west side (where strong currents bring nutrients and the fish life concentrates) and gently sloping coral gardens on the east, making it one of the most varied dive environments in Southeast Asia. The park is closed from mid-May to mid-October each year to allow the reefs to recover from the tourist season. Liveaboards run from Khao Lak, about 50 miles (80 km) to the south, and typically spend 2–3 nights in the park.
Site highlights include Elephant Head Rock, a scatter of granite boulders with swim-throughs that attract dense schools of fish; Christmas Point on the northern Similan islands, known for leopard sharks resting on the sandy bottom; and Donald Duck Bay on Koh Miang, which offers exceptionally calm conditions for newer divers and snorkellers.
Surin Islands and Richelieu Rock
Further north than the Similans, the five islands of the Mu Ko Surin Marine National Park offer some of the most pristine shallow water snorkelling in Thailand. Ao Mae Yai Bay on Surin Island is famous for clownfish in large anemones — a Nemo moment that snorkellers rank among their most memorable. The waters around the Surin Islands are also one of the most reliable spots in Thailand to snorkel alongside manta rays on an organised tour, particularly from January to April. A liveaboard itinerary that combines the Similans, Surin Islands, and Richelieu Rock is the definitive Andaman Sea diving experience — most serious divers rank it ahead of anything else Thailand offers.
Koh Phi Phi and Maya Bay
The Phi Phi Islands — located about 28 miles (45 km) southeast of Phuket — are primarily famous above the waterline, but the diving and snorkelling here are genuinely excellent, particularly at the two southern pinnacles Bida Nok and Bida Nai. These sites regularly deliver leopard sharks on the sandy bottom, large schools of barracuda, and, on lucky days, whale sharks passing through. Visibility here is typically 10–25 metres (33–82 feet).
Maya Bay — which became globally known after the filming of The Beach in 1999 — was temporarily closed in 2018 after the reefs inside were critically damaged by boat anchoring and tourist volume (over 5,000 visitors per day at peak). It has since reopened with strict controls: visitor numbers are capped, no anchoring on reef is permitted, and banned sunscreen is actively enforced here. Research by Chulalongkorn University and the DMCR found that oxybenzone concentrations in Maya Bay had reached levels known to cause coral stress and larval deformities before the closure — data that directly influenced Thailand’s 2021 sunscreen ban. The bay’s snorkelling has visibly improved since the recovery period.
The King Cruiser Wreck
For divers who want something structurally different from reef diving, the King Cruiser Wreck is Thailand’s most famous shipwreck and a compelling dive. On 4 May 1997, a Japanese-built twin-hulled car and passenger ferry on the daily Phuket-to-Phi Phi Islands route struck the submerged rock at Anemone Reef. The vessel sank slowly — over about two hours — allowing all 561 passengers to be safely evacuated. It has been nicknamed the “Thaitanic” by the dive community, partly for its scale (the wreck is 85 metres / 279 feet long and sits upright on the seabed) and partly because it is genuinely hard to see the whole thing in a single dive.
The wreck now sits at a depth of 18 to 33 metres (59 to 108 feet) and is accessible only to Advanced Open Water divers with a minimum of around 20 logged dives. Penetration of the interior is prohibited due to structural collapse. Marine life is extraordinary: large schools of yellow snapper and fusiliers cloud the mid-water, lionfish and scorpionfish camouflage themselves against the rusting hull, and at least one resident turtle has been documented beneath the wreckage. Night dives here are particularly atmospheric. The wreck is about 14 miles (22 km) east of Phuket and is best combined with dives at nearby Shark Point — a marine sanctuary famous for leopard sharks resting on the sandy bottom — and Anemone Reef, the same rock the King Cruiser struck in 1997.
Phuket (Day-Trip Diving for All Levels)
Phuket is the best base for beginner divers primarily because it offers the most choice, the most infrastructure, and the most consistently accessible sites regardless of experience level. A first dive from the shore or a day-boat can take you to sites like Racha Yai (clear water, turtles, good visibility) or Koh Doc Mai (excellent wall dive and macro life in two small caverns at 13 and 18 metres / 43 and 59 feet). The introductory experience on Phuket — a pool session followed by a shallow bay dive — costs around 1,500–2,000 Baht and is an excellent way to decide whether full certification is for you.
Koh Lanta, Hin Daeng, and Hin Muang
Off the coast of Koh Lanta, about 47 miles (75 km) southeast of Krabi, two deep-water pinnacles rise from the Andaman Sea floor: Hin Daeng (Red Rock) and Hin Muang (Purple Rock). These are among the most impressive dive sites in Thailand for experienced divers — both for the sheer drama of the walls (Hin Muang drops to over 70 metres / 230 feet) and the pelagic life they attract. Manta rays and whale sharks are encountered here with notable frequency from February to April. Because of the depth, current, and distance from shore, these sites require a minimum of Advanced Open Water certification with at least 20 dives. Day trips from Koh Lanta take approximately 90 minutes to reach both sites.
Koh Samui and the Gulf Coast
Koh Samui is less celebrated as a dive destination than the Andaman side, but it serves as a practical gateway to the Gulf’s best sites: Sail Rock, Chumphon Pinnacle, and the smaller pinnacles of the Ang Thong Marine National Park. Snorkelling from Koh Samui itself is average — the fringing reef around the island has suffered from development. The better approach is a day trip or a short boat journey north to Koh Tao for snorkelling, or to Sail Rock for diving. Sail Rock sits roughly 14 miles (22 km) northeast of Koh Phangan and is a deep offshore pinnacle that divides its time between schools of chevron barracuda and hunting whale sharks in the season window.
Ao Nang Bay (Krabi)
Ao Nang is the practical snorkelling and introductory diving hub for travellers based in Krabi. The bay itself has around ten dive shops and offers day trips to the outer Phi Phi Islands, Koh Dok Mai, and Koh Bida Nok within easy reach. The snorkelling directly off the beach at Ao Nang is modest — the real snorkelling is on the boat tour destinations. The atmosphere here is easier and more relaxed than Laem Phra Nang (which has technically better beaches but carries heavier tourist traffic), and it is a solid base for multiple days of island-hopping snorkel trips if you are not diving.
Kata Beach (Phuket)
For snorkellers staying in Phuket who do not want to organise a boat tour, Kata Beach — about 17 miles (27 km) south of Patong — offers the best accessible snorkelling on the island’s main coast. The northern headland of Kata Beach drops into a mixed hard and soft coral reef in 3–5 metres (10–16 feet) of water that is reachable from the beach with basic rental gear. It is uncrowded relative to the busier Phuket bays, the water is calm in the November-April season, and it is free to access.
A Note on Booking Liveaboards
The Similan Islands park has a fixed season (typically mid-October to mid-May) and liveaboard spaces fill weeks or months in advance during the February-April peak window. If you are planning a trip timed specifically for whale shark season at Richelieu Rock, book your liveaboard berth before you book your flights. Most operators accept a deposit to hold a space. Departures are typically from Khao Lak (about 50 miles / 80 km north of Phuket) or directly from Phuket’s Chalong Bay. The journey time from Khao Lak to the northern Similan Islands is around 3 hours (50 miles / 80 km by sea). Budget around 18,000–35,000 Baht for a 4–5 night itinerary depending on the vessel and season, with national park fees of approximately 700 Baht per person per day added on top.
Thailand has always over-delivered for divers and snorkellers — it is one of the few places in the world where you can do your first-ever dive one day and be planning a whale shark liveaboard the next. The reefs have faced pressure in recent years, and they need visitors who understand that. But for those who show up informed, dive with purpose, and leave the sunscreen on the shore, the underwater world here remains as extraordinary as its reputation suggests.