Best Thai Islands for Diving: Sites, Courses & When to Go

Thailand remains one of the world's more affordable places to learn to dive, with PADI Open Water courses running roughly ฿9,000–14,000 against ฿30,000 or more for the equivalent in Europe, and the country pairs that affordability with genuinely strong marine biodiversity across two distinct coastlines. The Gulf of Thailand and the Andaman Sea offer different enough diving that a serious trip can reasonably justify visiting both, rather than treating one as a lesser substitute for the other.

Koh Tao is the global centre of affordable certification, with over 60 dive schools competing on a small island where the water itself does most of the selling — shallow, sheltered bays for beginners working their way through confined-water training, and enough advanced sites nearby (Chumphon Pinnacle and Southwest Pinnacle among them) to keep experienced divers coming back after certifying. Sail Rock, roughly equidistant between Koh Tao and Koh Phangan, threads a genuine chimney passage through a submerged rock formation and is widely considered the single best dive in the Gulf. Divers hoping for a whale shark encounter should target April to June, when sightings cluster most reliably around Chumphon Pinnacle; a smaller second wave passes through in October, though with far less consistency. Night dives off the island turn up giant morays and sleeping reef sharks tucked into crevices the daytime traffic never disturbs.

The Similan Islands sit within reach of a day boat from Khao Lak and are regularly ranked among Asia's top five diving destinations, with cleaning stations drawing manta rays and whale sharks from January through May, and underwater scenery dramatic enough to justify the trip before the marine life even factors in.

Richelieu Rock, near Koh Surin, appears on more than one global top-ten dive site list, with manta rays and whale sharks passing through from January to April — a serious detour for anyone building a dedicated dive trip around the Andaman's best single sites rather than the most convenient ones.

At the Andaman's southern tip, within Tarutao National Marine Park, Koh Lipe offers excellent shore and boat diving with visibility regularly reaching 15–25 metres, protected blacktip and leopard shark populations, and sea turtles as a reliable sighting rather than a lucky one. At least five established dive shops operate on the island, running day trips out to the comparatively undisturbed Butang island group — a genuine step up from the more heavily dived sites closer to Phuket.

For a more immersive trip, liveaboards from Khao Lak and Phuket run three-to-seven-day Similan and Surin itineraries from ฿15,000–40,000 all-inclusive, and the Mergui Archipelago, reached via Ranong, offers experienced divers a genuinely remote alternative for anyone who has exhausted the more established sites.

Season splits by coast: November to April for the Andaman, April to October for the Gulf. Water stays warm year-round at 28–30°C, which removes one of the usual barriers to diving somewhere unfamiliar.