Koh Bon

Unlike the granite islands that make up the rest of the Similan chain, Koh Bon is reported as limestone, and it splits into two distinct profiles on the same rock. West Ridge slopes from a shallow fringe down to a wide plateau of table coral, typically 15 to 25 metres, with the site's outer edges falling past 40. Divers usually enter up-current and drift the length of the ridge; the eastern face runs as a wall, the western side eases into a gentler, coral-covered slope. Koh Bon Pinnacle sits further out and deeper, a current-swept structure of overhangs and crevices that suits a more experienced group.

The reason most boats stop here at all sits over the ridge: a cleaning station where reef manta rays circle to have parasites picked off by wrasse. A manta can hold position for minutes at a time, close enough to watch the whole routine, but the rays come and go with the season and the current, and a dive here can just as easily pass with only trevally, octopus and reef sharks for company.

Koh Bon is one of only two places in the Andaman Sea where reef manta rays show up often enough to plan a dive around, the other being Hin Daeng and Hin Muang far to the south. On the same liveaboard route, Richelieu Rock draws the crowds and Similan proper draws the granite boulder swim-throughs; Koh Bon draws people hoping for a manta and gets a quieter dive either way. Divers who have done both describe it as calmer and less crowded than Richelieu, with good small life even on a day when no ray turns up: an octopus working the reef, reef sharks over the sand, big schools of trevally in the blue.

None of that makes a sighting certain. A manta showed up for one 2025 group and not for another on the same week's itinerary, and one earlier visitor did a full liveaboard without a single encounter before coming back specifically to try again. The site earns its reputation on frequency, not on guarantee.

Currents on the west ridge can build quickly, and downcurrents here are reported strong enough to sweep a diver off the wall into open water; treat the current briefing seriously and stay close to your guide. It is easy, watching a manta at the cleaning station, to lose track of bottom time, so run your computer on a schedule and carry a surface marker buoy for the ascent.

No dive centre operates exclusively at Koh Bon. It sits on the route of multi-site liveaboard and day-boat operators mostly based around Khao Lak, visited on the same trip as Koh Tachai and Richelieu Rock or, in season, on its own by day boat. Open Water is enough for the shallow fringe reef, but the main ridge profile calls for Advanced Open Water given its depth and current, and the deeper pinnacle work past 30 metres needs a Deep specialty or equivalent.