The waters south of Koh Mak hold the best dive sites in eastern Thailand. Hin Rap and Hin Luk Bat are pinnacle dives covered in soft coral, with schooling fusiliers, the occasional whale shark in March and April, and almost always reasonable visibility. The wreck of the HTMS Chang, a former US Navy landing ship deliberately sunk in 2012 as an artificial reef, sits in 30 metres of water south-east of the island and is now one of the largest diveable wrecks in the Gulf.
A few small dive shops on Koh Mak — including the long-running BB Divers and Koh Mak Divers — run two-tank trips for around 3,000 baht. Snorkellers can join the same boats for less.