Diving in Thailand: Similan Islands, Koh Tao & Richelieu Rock
Thailand has been pulling divers for forty years and the reasons are still the same: the water is warm, the reefs are remarkable, the prices are honest, and the logistics are easy. I've done a lot of expensive diving in difficult-to-reach places. Thailand is the destination I keep coming back to when I want great diving without the hassle.
But Thailand isn't uniform. The country has two coastlines — the Andaman Sea to the west and the Gulf of Thailand to the east — and they operate on completely different seasonal cycles, different marine life profiles, and different price points. Understanding which side to dive and when to go is the first decision you'll make, and this guide will walk you through it.
Understanding Thailand's Two Dive Seasons
This is the most important thing to know before booking: Thailand's two dive coasts have opposite monsoon seasons.
Andaman Sea (west coast): Dive season November through April. The southwest monsoon (May–October) closes the Similan Islands National Park completely and brings rough conditions to Koh Lanta, Phi Phi, and the Mergui Archipelago. Richelieu Rock and all the headline Andaman sites are inaccessible May through October.
Gulf of Thailand (east coast): Dive season year-round, with best conditions September through June. Koh Tao, Koh Samui, and the Sail Rock area dive well most of the year. The northeast monsoon affects the Gulf November through February but is generally milder and less disruptive than the Andaman monsoon.
Bottom line: If you want Similans or Richelieu Rock, you must go November through April. If you're coming for Koh Tao certification or just want good diving with schedule flexibility, the Gulf side works most of the year.
Best Time to Dive Thailand (Seasonal Breakdown)
November through January
The Andaman season opens in mid-October and is typically at its best by November. Whale shark sightings at Richelieu Rock and Koh Bon begin. Similan National Park opens. Water temperature is 27–29°C, visibility 20–30 meters. This is the beginning of Thailand's high tourist season on land as well — book early.February through April
Peak dive season for the Andaman. February and March are historically the most reliable months for whale shark sightings at Richelieu Rock. Similan liveaboards run full schedules. The Gulf side (Koh Tao) is also diving well. This is the best window if you can only come once.May through June (Gulf Only)
Andaman sites close. Koh Tao and Gulf dive sites continue. Some operators on Koh Tao report excellent visibility in May–June before the Gulf's own wetter period. Whale sharks are occasionally encountered around Sail Rock in the Gulf.July through September (Gulf Only)
Andaman is closed. Gulf diving continues with some disruption from the northeast monsoon. Koh Tao divers report good macro life and reef fish but variable visibility. This period sees fewer tourists and lower prices.October (Transition)
The Andaman season is about to open, liveaboard operators are returning from maintenance breaks, and anticipation is high. Some years, early-opening liveaboards catch excellent conditions in late October.Top Dive Sites in Thailand
Richelieu Rock, Surin Islands
The most famous dive site in Thailand and, by most accounts, one of the ten best in the world. An isolated underwater pinnacle in the Surin Islands Marine Park that rises from 35 meters to just below the surface. The density of marine life here is almost implausible: leopard sharks rest in crevices, enormous bumphead parrotfish cruise the top, whale sharks appear November through April, and the whole structure is carpeted in soft corals and surrounded by schooling fish. Liveaboard only — no day trips. [Explore Richelieu Rock →](/dive-sites/)Koh Bon Pinnacle
A seamount between the Similan Islands and Surin Islands that concentrates pelagic life. Manta rays are the headline — during peak season, the cleaning stations at Koh Bon see multiple mantas on most dives. Oceanic whitetip sharks appear occasionally. Hammerheads pass through. This is a current-heavy, intermediate dive, but the payoff is substantial. [Explore Koh Bon →](/dive-sites/)Similan Islands: Elephant Head Rock
The signature dive in Similan National Park. A maze of granite boulders creates swim-throughs and archways descending to 30 meters. Inside the channels: barracuda schools, leopard sharks, marble rays, and clouds of anthias. The topography here is unique — the Andaman coast's granite island formations don't exist in the Gulf. [Explore Similan Islands dive sites →](/dive-sites/)Koh Tao: Southwest Pinnacle
Koh Tao has over 25 named dive sites and is well-suited to most skill levels. Southwest Pinnacle is the island's best — a series of pinnacles descending to 33 meters with resident whale sharks (year-round, though sightings aren't guaranteed), leopard sharks on the bottom, and blacktip reef sharks circling the shallows. More reliably impressive than almost anywhere else accessible as a day dive. [Explore Koh Tao dive sites →](/dive-sites/)Sail Rock, Gulf of Thailand
A seamount in the middle of the Gulf between Koh Tao and Koh Phangan, accessible as a day trip from either island. A dramatic chimney runs through the center of the rock from 18 meters to the surface — you ascend through the interior surrounded by hundreds of fusiliers. One of the most unusual dive experiences in the Gulf. Whale sharks are occasionally seen here in the cooler months. [Explore Sail Rock →](/dive-sites/)Koh Lanta: Hin Daeng and Hin Muang
The best dives on the Krabi/Koh Lanta side of the Andaman. These two remote pinnacles, reachable as a long day trip from Koh Lanta, are among the deepest and most spectacular in Thailand. Hin Muang descends to over 70 meters on the deepest side and is considered one of the most vertical walls in the region. Whale sharks appear here as well during peak season. Open October through April. [Explore Hin Daeng and Hin Muang →](/dive-sites/)Shark Point (Hin Musang), Phuket
A set of three pinnacles near Phuket notable for consistent leopard shark sightings. The sharks rest on the sand between pinnacles, sometimes in groups, making this one of the most reliable sites in Thailand for close encounters with large sharks. Accessible as a day trip from Phuket year-round (with some seasonal variation). [Explore Phuket dive sites →](/dive-sites/)Koh Tao: The Budget Certification Capital
Koh Tao deserves special attention because it genuinely changed the global dive industry. Since the 1990s, the island has been the cheapest place on Earth to get a PADI Open Water certification — currently around $280–350 USD all-in, including course materials, pool sessions, and four open water dives. The same certification runs $400–600+ in the US, Europe, or Australia.
The reason: competition. Koh Tao has over 50 dive shops on an island you can walk across in an hour. That density drives prices down. The better shops invest the savings into more instructor time per student and better equipment. The worse shops are, frankly, racing to the bottom. Research matters here more than at most destinations — check instructor-to-student ratios and read recent reviews.
For already-certified divers, Koh Tao offers fun diving at $25–40 for 2–3 dives, specialty courses at significant discounts vs. home, and a lively dive community at every bar on the beach.
Similan Islands Liveaboards: What to Expect
The Similan Islands are 80 kilometers offshore from the Khao Lak coast. Day trips exist but are long and exhausting. Liveaboards are the right way to dive the Similans and Surins.
Typical itinerary: 3–5 nights, departing from Khao Lak or occasionally Phuket. Sites covered include the Similans (sites 1–9), Koh Bon, Koh Tachai, and Richelieu Rock. 3–4 dives per day, 12–18 dives total.
Cost: $350–700 USD for a 3-night/4-day trip. Premium vessels with fewer divers and better food run $600–1,000. Budget vessels are available around $300 but inspect carefully before booking.
Timing tip: The Similan season runs mid-October through mid-May. The Park closes completely during the monsoon for reef conservation — this is enforced. Book Similan liveaboards well in advance for February and March, when whale shark sightings peak and vessels fill completely.
Visa and Travel Logistics for US Passport Holders
Visa: US passport holders receive a 30-day visa exemption on arrival in Thailand, with the option to extend once for 30 additional days at an immigration office. No prior application required. The exemption covers both tourism and diving activities.
Getting There: Bangkok's Suvarnabhumi Airport (BKK) is the main hub, with direct flights from major US gateways including LAX, JFK, and Houston connecting through partner airlines. For the Andaman coast (Similans, Phuket, Koh Lanta), fly into Phuket (HKT) directly from Bangkok. For Koh Tao and Gulf diving, fly into Koh Samui (USM) or Surat Thani (URT) and take a ferry.
Domestic Connections: Bangkok to Phuket is a 1.5-hour flight on multiple daily schedules. Bangkok to Koh Samui is similar. The island ferry services are well-developed — high-speed catamarans connect Koh Samui, Koh Phangan, and Koh Tao several times daily.
Health: No vaccines required for entry. Hepatitis A and typhoid are sensible precautions for most Thailand trips. Malaria risk is considered low in tourist areas; the Thai-Myanmar border region is higher risk and relevant if you're doing Mergui Archipelago liveaboards.
Currency: Thai Baht (THB). ATMs are everywhere in tourist areas. Most dive shops accept credit cards but add a 3–4% surcharge — cash saves money on larger transactions.
Typical Costs
Thailand remains one of the most affordable diving destinations globally, particularly on the Gulf side.
Koh Tao Day Diving: $25–40 USD for 2–3 dives including equipment. Boat, dive guide, and tanks included. Nitrox available at most shops for a small surcharge.
Koh Tao Courses: Open Water Certification: $280–350. Advanced Open Water: $250–300. Rescue Diver: $250–300. Divemaster: $700–1,000 (exceptional value compared to any Western country).
Phuket / Koh Lanta Day Diving: $60–90 for 2–3 dives. The Andaman sites require faster boats and longer transits, which raises prices above Koh Tao.
Similan Liveaboards: $350–1,000 for 3–5 nights depending on vessel. Mid-range boats in the $450–600 range offer good quality.
Land Accommodation: Budget guesthouses on Koh Tao and Koh Lanta start around $20–40/night. Mid-range bungalows and hotels run $60–120. Phuket beach resorts command premium rates; staying in Ao Nang (Krabi) or Khao Lak for Similan trips saves significant money versus Phuket itself.
Realistic Daily Budget:
- Budget: $70–100/day (hostel, 2 dives, street food)
- Mid-range: $150–200/day (mid-range hotel, full dive day, restaurant meals)
- Comfortable: $250–350/day (quality resort, private guide option)
Certification Requirements
- Open Water: Sufficient for Koh Tao, most Gulf sites, and beginner Andaman sites like Phi Phi and Koh Racha.
- Advanced Open Water: Recommended for Similan sites, required by some operators for Richelieu Rock and the deeper Andaman pinnacles (Hin Muang, Koh Bon). The 30-meter depth certification component of AOW is relevant here.
- Deep Diver Specialty: Useful for Hin Muang (which can be 40+ meters on the deep side).
- Nitrox: Not required but widely available and worth it for multi-dive days.
Marine Life by Season
November through February (Andaman Peak):
- Whale sharks beginning to appear at Richelieu Rock and Koh Bon
- Leopard sharks consistent at Shark Point and Koh Bon
- Manta rays at Koh Bon and Hin Daeng
- Water 27–29°C, visibility 20–30 meters in Andaman
- Whale shark sightings peak February–March at Richelieu Rock
- Bumphead parrotfish spawning aggregations at some Similan sites
- Barracuda schools, schooling giant trevally
- Whale shark encounters also possible in the Gulf at Sail Rock
- Gulf whale sharks occasional, not predictable
- Leopard sharks year-round at Koh Tao sites
- Blacktip reef sharks year-round at Southwest Pinnacle
- Macro life excellent: nudibranchs, frogfish, octopus
- Seahorses at several Koh Tao sites
Recommended Trip Length
Koh Tao Only: 5–7 days is ideal. Enough time for a certification course plus fun dives, or 8–10 dives if you're already certified.
Phuket / Andaman Day Diving: 5–7 days. Day trips to Shark Point, Phi Phi, Racha Noi, and possibly Hin Daeng.
Similan Liveaboard: The liveaboard itself is 3–5 nights. Add 2–3 days in Phuket or Khao Lak before/after for decompression and acclimatization.
Combined Trip: 14 days combining a Similan liveaboard (5 days) with Koh Tao (5 days) and transit days is a classic Thailand diving itinerary. Both sides of the country, two entirely different experiences.
Frequently Asked Questions
Q: Is Koh Tao still worth it for certification in 2026? A: Yes. The price advantage versus Western countries remains significant — $300 in Thailand versus $500–700 elsewhere. Quality varies by shop, so research matters. Look for shops with small instructor-to-student ratios (maximum 4:1 for pool, 4:1 for open water) and recent positive reviews.
Q: How reliable are whale shark sightings at Richelieu Rock? A: Not guaranteed, but better than almost anywhere else. February and March are historically the most productive months. Operators track sightings and share information across the fleet — if a whale shark is at Richelieu, every liveaboard will be redirected there. Roughly 60–70% of liveaboards that run during peak season report at least one whale shark encounter.
Q: Can I dive the Similans and Koh Tao on the same trip? A: Yes, but the logistics require thought. The Similans are on the Andaman coast (fly into Phuket or go to Khao Lak). Koh Tao is in the Gulf (ferry from Surat Thani or fly to Koh Samui). Getting between the two coasts involves crossing the peninsula — typically a flight from Phuket to Bangkok to Koh Samui, or a bus/ferry combination through Surat Thani. Allow a full day of travel.
Q: What's the marine life comparison between Andaman and Gulf sides? A: Andaman wins for pelagics — whale sharks, manta rays, and the sheer scale of schooling fish at Richelieu Rock and Koh Bon. Gulf wins for accessibility, year-round availability, and budget. Koh Tao's marine life is genuinely good for a day-trip destination; it's just not in the same league as Richelieu Rock for jaw-dropping encounters.
Q: Is Thailand diving safe? A: Generally yes. Thailand has a mature dive tourism industry with well-established operators. The biggest risks are the same as anywhere: currents at more advanced sites, depth-related issues if certification limits aren't respected, and occasional minor boat incidents in rough weather. Dive with operators who brief sites properly and know the conditions.