Sail Rock
Depth is the range from the top of the site to its usual maximum; "recommended cert" is the mainstream-agency level operators usually ask for, not a legal rule. Currents and visibility change daily — treat everything here as planning ranges.
Sail Rock stands alone in the middle of the gulf, a granite sail above water and a fish magnet below it, widely rated the best dive in the Gulf of Thailand. Its party trick is the Chimney, a vertical shaft through the rock that divers enter near 18 m and exit around 6, rising through a tube of stone with fish flickering past the openings. Outside, the monolith disappears under snapper, fusiliers, and big-eye trevally so dense they behave like weather, with queenfish and king mackerel raiding the edges and whale sharks calling unannounced. Batfish follow divers around like hired guides.
The moment you come for: The Chimney swim-through, bait-ball density on a lone monolith, regular whale-shark cameos
Topography: Granite monolith rising from around 30-34 m to above the surface
Entry: Roughly midway toward Koh Phangan and dived by boats from Koh Tao, Phangan, and Samui; expect company.
Know before you splash
Hazards: An overhead shaft with two-way traffic, mid-sea exposure, and busy surface conditions.
Conservation: Heavy multi-island boat traffic; moorings, not anchors, and patience at the Chimney entrances.
Nitrox: Worth it here — profiles sit in the range where enriched air meaningfully extends no-stop time (with the training to use it).
Who you'll meet
Giant Grouper
Giant Trevally
Great Barracuda
Pinnate Batfish
Whale Shark
For photographers
Wide angle: ★★★★★ · Macro: ★★☆☆☆ — editors' 1–5 rating of what this site gives each style of shooting