Best Dive Sites in Thailand: From Koh Tao to the Similans

Thailand's diving is split between the Gulf of Thailand (east coast) and the Andaman Sea (west coast) — two bodies of water with distinct characteristics, different peak seasons, and different marine life. The Gulf is generally calmer, more accessible year-round, and home to Koh Tao, which certifies more divers than almost anywhere on earth due to its shallow, visibility-clear training sites. The Andaman offers larger, more dramatic dive sites including the world-class Similan Islands and Richelieu Rock, often cited as one of the top five dive sites on earth.

Koh Tao's best sites include Chumphon Pinnacle (strong currents, large schools of fish, whale shark sightings possible), Southwest Pinnacle (coral-covered rocks with leopard sharks), and HTMS Sattakut (wreck dive, 30m). All three require Open Water certification minimum; Chumphon demands some experience with current diving. The Similan Islands (reachable by liveaboard from Khao Lak, open November–April) feature extraordinary visibility — often 30m — around granite boulder formations. Richelieu Rock in the Surin Islands is the jewel: a submerged pinnacle carpeted in soft corals and reliably visited by whale sharks from February to April. For budget accessible diving without a liveaboard, Koh Phi Phi's Bida Nok and Bida Noi sites off the southern tip have excellent coral gardens and blacktip reef sharks. Hin Daeng and Hin Muang, south of Koh Lanta, are considered the Andaman's best accessible sites with regular manta ray encounters.

Choosing between a liveaboard and day-boat diving changes what the trip actually feels like, not just what it costs. A liveaboard sleeps aboard the boat for several days, reaching sites like the Similans and Richelieu Rock at first light before day-trip boats arrive and fitting in three or four dives daily without the transit time eating into the schedule — the standard way to dive these remote sites properly rather than as a rushed add-on. Day boats from Khao Lak or Phuket can reach the same general area but spend a meaningful chunk of the day travelling there and back, generally fitting in two dives rather than four, and skip the early-morning and after-hours windows when a liveaboard has the site closest to itself. For anyone serious about the Similans or Surin Islands specifically, the liveaboard is the format the sites reward.

Current strength is the practical skill filter behind several of Thailand's best sites, and it's worth an honest self-assessment before booking. Chumphon Pinnacle and Richelieu Rock both regularly run genuine current that requires comfort with drift diving and controlled buoyancy under load — not a place to attempt current diving for the first time. Sites like Bida Nok and the shallower Similan boulder formations are far more forgiving, making them sensible choices for divers who are certified but still building the specific skill of managing moving water rather than the calm, static conditions most Open Water training happens in.

The marine park status covering the Similan and Surin Islands does real conservation work, not just naming — commercial fishing is restricted within park boundaries, and the parks close entirely for several months each year to let reef and fish populations recover from the diving season's pressure. That closure is why liveaboard trips to these sites are strictly seasonal rather than year-round, and it's part of why the visibility and coral condition at reopening each November tend to be noticeably better than at sites without the same protection.

A simple framework helps when comparing this list against your own certification and experience: newly certified divers do best starting at Koh Tao's calmer sites or Koh Phi Phi's reef gardens, divers with a season or two of experience are ready for the Similans' boulder formations, and only genuinely current-comfortable divers should book Chumphon Pinnacle or Richelieu Rock as anything other than a guided dive with a briefing that matches the conditions to their logged experience.