Sail Rock Dive Site, Koh Tao: Chimney, Depth & Whale Sharks

Sail Rock, known in Thai as Hin Bai, is widely regarded as the best dive site in the Gulf of Thailand. A big mushroom-shaped granite rock rises about 10 m out of open water roughly midway between Koh Tao and Koh Phangan, closer to Phangan. Below the surface the walls drop into the blue with pillars, ledges and cavernous overhangs.

A Sail Rock dive from Koh Tao is a full-day outing: about 1 to 1.5 hours by boat each way, usually with two dives and lunch. It is worth waiting for a calm day and good visibility.

Usual route: Descend along the outer walls or an outlying pinnacle, work round the main rock, enter the Chimney at about 5 m and exit around 18 m, then finish shallow. Drawing based on local dive briefings: approximate, not to scale and not for navigation. Your guide's briefing on the day always comes first.

Diving Sail Rock: the route

Most groups descend along the outer walls or one of the outlying pinnacles, work around the main rock, then enter the Chimney from the top at about 5 m and exit around 18 m. Some guides brief it the other way round, so listen to your briefing. The dive finishes shallow on the rock.

Think of the dive in three parts. First the outside: the walls, pillars and overhangs, and the big schools that circle the rock. Then the Chimney, which most divers remember for years. Then the shallow finish on the rock itself, where there is plenty of colour and small life to watch while you complete your safety stop.

Two dives are standard on the day, and some trips add a third on the way back to Koh Tao. Boats usually run to Sail Rock a few times a week in season, so ask your dive centre which day suits you.

The Chimney

The signature is the Chimney, a vertical swim-through inside the rock. Divers go single file through a shaft filled with glassfish, sponges and groupers. The top opening is around 5 to 6 m and the exit is around 12 to 18 m, though exact depths vary with who you ask.

It is narrow, so good buoyancy matters. Pause to watch the fish, and bring a torch.

Inside, the light filters down from the top and the glassfish fill the shaft, so it feels like swimming through a living column. Keep your fins close to your body and avoid kicking the walls. If your group is large, hang back a moment and let the diver ahead clear the exit. A wide-angle lens and a torch both earn their place here.

Underwater layout

The main pinnacle has smaller clusters off its south-west and north sides, and outlying pinnacles to the south, east and west. The east pinnacle tops out at 16 to 18 m, standing on sand below 30 m. The west cluster stands in about 32 m with its top at about 18 m. A ledge at about 12 m is calm enough for training.

Walls are covered with corals, sponges, giant black coral and anemones down to about 18 m. The bottom is steeper on the north side, where sand slopes to 40 m. Experienced divers may find a secret pinnacle off the north side with a guide.

Marine life at Sail Rock

This is big-fish water. Barracuda, big-eye trevally, snapper and fusiliers gather in schools that can number in the thousands, with queenfish and Spanish mackerel around them.

Residents: honeycomb and giant groupers, moray eels, batfish, bannerfish and anemonefish.
Visitors: whale sharks at certain times of year, giant barracuda, and occasional reports of bull sharks.
Macro: nudibranchs, wart slugs, scorpionfish, shrimps and crabs.

The schools are the show. Trevally wheel around the rock like weather. Moray eels hide in the crevices, and scorpionfish sit perfectly camouflaged on the rock, so look twice before you put a hand down.

Conditions and best time to dive

Conditions are variable. The mainland often shelters the rock, but it is open to wind-driven waves and current, so dive centres time trips for calmer periods. Visibility is roughly 10 to 40 m, and can be limited on one side of the rock. Calmer water is often cited from May to September, with good visibility in March and April too. Trips are cancelled in rough weather.

Tips from local divers

Take a torch for the Chimney and crevices.
Descend through the Chimney single file with good buoyancy control.
Wide-angle suits the schools, close-up suits the crevices.
Choose a calm, clear day and ask your dive centre when trips run.
Look closely for camouflaged scorpionfish.

Safety notes

Depth reaches 40 m on the northern sand, so Advanced certification is ideal, though Open Water divers can dive the shallower parts with a guide. Watch for current, wind-driven waves, crowded groups and the narrow Chimney.

Boat days to Sail Rock can be rough in the monsoon weeks, so check the forecast and your dive centre's advice. Listen carefully to the briefing, especially about entering the Chimney and where to surface.