Leopard sharks are pretty 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. .

Above the waterline there is a huge hole 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 currents are strong it’s not worth the effort. But, if the water is clear, you’re good with your breathing gas, 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 18 meters and the other pinnacle comes only up to about 24 meters. The sandy areas around the rock are well over 45 m 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. 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.

A ridge runs between the two pinnacles, though the eastern rock is the one most people dive as there are boat moorings making it easier for divers to descend in a strong current. If you have the gas 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.

A ridge runs between the two pinnacles, though the eastern rock is the one most people dive as there are boat moorings making it easier for divers to descend in a strong current. If you have the gas 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.

Koh Tachai also boasts a breathtaking sandy beach on its northeastern shore; It’s a popular anchorage when northeast winds have calmed down in March and April. The beach is one of the prettiest anywhere though it’s not always open.

Some boats offer night dives 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.

Nearly every imaginable variety of bony fish is found here, including many species of butterfly fish, wrasse, damsel fish, lion fish and a host of other reef dwellers. Among the most abundant species are scorpion fish, which are amazingly adept at blending in with their surroundings. Be especially careful where you put your hands, because many things that look like rocks are not really rocks! A number of eel species live here as well, including giant, white-mouth, fimbriated, snowflake, bar-tail and zebra morays. Schooling fish like fusiliers and snapper are also prevalent, in addition to open water predators like rainbow runners, mackerel, and barracudas. One schooling fish deserves special mention–a huge school of bigeye jacks can often be seen on the up-current side of the rock, where they often circle divers en masse, creating a living maelstrom of fish. Schooling chevron barracudas are seen almost all the time.

Divemasters are now quite good at spotting ghost pipefish, Harlequin shrimp, and tiger-tailed seahorses–if you don’t see something weird here you just aren’t paying attention. Cuttlefish hover around, usually in the autumn, but very often at other times of the year as well. Octopus is common, and lionfish as well as different kinds of anemone fish are easily spotted.
Richelieu Rock is a world-class dive site, and if you don’t like diving here, you simply don’t like diving! You can dive here over and over and see something new. The currents change constantly, and it’s not always an easy dive. But, if you listen to your dive guide and his or her briefing, you can learn how to hide from the currents and have a very enjoyable dive all the time. Many boats will spend the entire day here so you can get a full appreciation of the variety of conditions and marine life at this fantastic dive site.

Note:Recently there have been postings (even on PADI’s website) that Richelieu Rock was named by Jacques Cousteau’s team when they were filming in Thailand and the Andaman Islands in 1988 and 1989. However, Richelieu Rock was named long before Mr Cousteau visited these waters so this story is simply inaccurate no matter how romantic it sounds. There were European explorers all over Southeast Asia from the 15th to 19th centuries and many European names exist for islands and rocks in Thailand as well as in the Mergui (Myeik) Archipelago in Burma. By the way, Cousteau was denied permission to go to Burma when his team was here, but he did do a little-known film about the Andaman Islands called The Invisible Islands. It’s worth watching if you can find a copy.