Koh Tachai

What makes Koh Tachai special

The dive site sits roughly 400 metres south of Koh Tachai island itself, a submerged rocky ridge often called the Twin Peaks or simply the Pinnacle. A dome-shaped granite boulder rises to around 12-15 metres, ringed by smaller rocks, sea fans and soft coral before the slope runs out into sand at 30-35 metres. Massive boulders create swim-throughs and current shadows where fish stack up and where you tuck in to catch your breath.

What sets Koh Tachai apart from the calmer Similan reefs to the south is its exposure. It stands alone in open water, so it catches current from every direction, and that current is the whole point. The nutrients it carries feed the plankton, the plankton feeds the food chain, and the food chain draws in the animals divers travel across the world to see. It is regularly rated among the top dive sites in Thailand for exactly this reason.

One honest note on access: Koh Tachai island has been closed to the public since 2016 to let it recover from tourism pressure, but the dive sites around it remain open. You are diving the water, not landing on the beach.

What you'll see

Koh Tachai is a big-animal site with a strong supporting cast. When the current runs, the fish action becomes genuinely spectacular — this is one of the best places in the Andaman to watch a barracuda tornado, hundreds of fish turning in a slow, hypnotic cylinder over the rocks.

Keep one eye on the reef and one on the blue. The reef holds the residents; the blue holds the reason you came.

• Manta rays — the headline draw, most likely Feb-Apr when plankton-rich current moves through the cleaning stations
• Whale sharks — a genuine but never guaranteed possibility in the same window
• Chevron barracuda and giant, bluefin trevally in hunting schools
• Dogtooth tuna and rainbow runners patrolling the current line
• Marble grouper, curious batfish, and dense clouds of fusiliers
• Soft coral, sea fans and reef fish across the boulders, with octopus and spiny lobster tucked into cracks

Conditions and who it's for

This is not a beginner site, and it is fairer to say so up front. The current at Koh Tachai is variable and frequently strong, and its defining trait is that it can change direction and strength during a single dive, seemingly at random. Local divers talk about the Green Monster — a cold, plankton-heavy current you can sometimes see coming as a greenish wall, bringing a temperature drop, murkier water and real push.

You want solid buoyancy, comfort with negative entries and reef hooks, and the discipline to stay with your guide and use the rocks for shelter. Divers with an Advanced certification and some current experience will get the most out of it and enjoy it; newer divers can be brought here in the right conditions, but it should be the crew's call on the day, not a fixed plan.

Visibility rewards and punishes the same mechanism: the current that brings the mantas can also cut visibility from a bright 30 metres to a murky 15. That trade-off is part of the deal, and part of why no two dives here are the same.

When to dive it

Koh Tachai is only diveable within Thailand's Andaman season. The Similan and Surin marine parks close from around 16 May to mid-October each year for ecosystem recovery, so the site is off-limits for roughly five months annually — build your trip around that window.

Within the open season, late October through early May all fish well. If mantas and whale sharks are your priority, aim for the February-to-April peak, when the shifting nutrient-rich currents fire up the cleaning stations at Koh Tachai and neighbouring Koh Bon. It is never a sure thing — these are wild animals over open water — but those months give you the best odds Thailand offers.

How you actually dive it

Because it sits well north of the day-boat reefs, Koh Tachai is realistically a liveaboard site — day trips from Khao Lak exist but are long and weather-dependent, and a liveaboard puts you on the pinnacle at the right state of tide rather than whenever a fast boat can reach it.

The plan is current-led. A good crew reads the flow, briefs a drift, and often drops you negative onto the pinnacle so you get down before the current sweeps you off. You work the up-current side and the boulder shelters, hang where the fish are stacking, and let the guide reposition you as the water shifts. On slack, calmer days you can circle the pinnacle more freely and enjoy the coral. Either way, watch your air and depth — it is easy to get absorbed in a barracuda school and look up to find you have drifted deep and far.

Koh Tachai: common questions