Koh Bon is located about 20km north of Similan Island #9 and features one of the only vertical walls in Thailand. The main dive site is on the southwestern point and has a step-down ridge that carries on to depths of over 45m. The eastern side of this ridge forms the wall and this is where most divers will enter the water and generally where the boats moor. The western side of the ridge is more of a gentle slope with coral bommies forming mushroom-like formations out of the finger corals. On the ridge itself, seafans of different sizes, shapes and colors grow, and schooling fish swim in the current feeding. This is generally where most divers will sit most of the dive, hoping for a sighting of the manta rays which frequent this dive site. These creatures seem to travel from Bon to Tachai and back again during the diving season from October until May.
Leopard sharks are common below the ridge on the sandy flats. Although the soft corals are not as high-profile as they are in the Similans, the colors of the corals are radically different and include shades of turquoise, yellow and blue, besides the more common pinks and purples. The colors are more pastel than the usual hard bright we see commonly other places.
Rays will sometimes be found in the deeper areas at depths for more experienced divers. The Koh Bon ridge is full of individual and schooling fish and some overhangs have places for smaller creatures to hide, but this is more of a fish and coral dive than a critter dive.
There is a hole above the waterline on the ridge and when a swell is running it pushes the water through the hole and the force of it makes white water below the surface making for a fun and exciting safety stop. Just don't get too close to the surface if the swell is big.
There is a pinnacle in the bay just to the north of the ridge. It's a deep rock, but if you're an experienced diver it's worth the short bottom times you'll get as it's a very special site. Not all boats visit it and if conditions are not good (like the current is running strong), it's not worth the effort. But, if the water is clear, you're good with your air, and the current is slack, it's a stunning rock. It looks like a monolithic temple built by some ancient culture.
The rock runs along an almost east to west course, the taller of the two pinnacles comes up to about 18m and the other pinnacle comes only up to about 24m. The sandy areas around the rock are well over 45m so beyond the range of most scuba divers. However, when the visibility is good you see all the way around the rock. Leopard sharks hang here, but it's also a good place to see smaller critters hiding in the rock. It's not dived as much as the ridge since it's deeper and the currents can make it more difficult so beginners have a tough time here. The pinnacle is fairly close to the main ridge and it's possible to swim to the ridge, though there is little reason to do so.
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 exiting as it gets here in Thailand.
Koh Tachai also boasts a breathtaking sandy beach on its northeastern shore; It's a popular spot to spend the night later in the season with the northeast winds have calmed down. The beach is one of the prettiest anywhere. There is a small ranger station there now to keep the place clean. It's a great place to hang out and feel like Robinson Crusoe.
Some boats do night diving on the western part of the island in a small bay or in front of the beach weather permitting. The main ridge is generally too deep and the currents too strong for reasonable and safe night diving.
John Williams has written, co-authored, or contributed to four diving guides on Thailand, the Mergui Archipelago, and the Andaman Islands. These are the award-winning Lonely Planet Diving & Snorkeling series, Periplus Editions Diving Southeast Asia, Asian Diver Scuba Guides and Singapore's Times Edition Diving Thailand. He has lived in Phuket and dived in Thailand's waters since 1987.