Koh Tachai
Twenty-five kilometers north of Koh Bon, Koh Tachai has an offshore underwater ridge that runs perpendicular to the island. Known as "Twin Peaks" this is considered to be one of the finest dives in the Kingdom of Thailand and is famous as a place to see not only the more common species of corals, fans and tropical fish, but larger animals such as rays, leopard sharks, nurse sharks and hawksbill turtles. Whale sharks and Manta rays make an appearance on a regular basis.
A ridge runs between the two pinnacles, though the southern rock is the one most people dive on as there are two boat moorings there which makes it easier for divers to descend in a strong current. If you have the air and the current is not too strong, it's worthwhile swimming to the other rock which comes up a bit shallower. However, most of the time the current runs perpendicular to the ridge making a long swim tiring.
It's heaven for the fish, though; they hunt and dart around, coming very close to divers, making the diving here about as exciting as it gets here in Thailand