Shark Fin Reef is located 1 km southeast of Similan Island 3 (Koh Pa-Yan) and extends all the way to Boulder City. It’s the largest dive site of the Similan Island with its approximately 1 kilometer long formation of reef and granite boulders and cliff-like drop offs. This site was named for the shape of its elongated ridge of 3 pinnacles rising above the surface at low tide which reminds many of shark fins. Most of the northern dive site consists of hard coral, soft coral and gorgonian sea fans with a maximum depth for scuba divers of around 20 meters. In the southern area the boulders make steep walls, a long canyon, platforms, cracks, and small caves to discover. Here depth goes as deep of 40 meters. A large swim-through makes a natural passage way from one side to the other.
The medium to strong currents at Shark Fin Reef go along the dive site and will give divers an unforgettable drift dive. With such good visibility, it is always worthwhile keeping an eye out into the blue for passing by manta rays and whale sharks, the famous Big Two at the Similans. Being such a long site, you will have to decide which side you would like to dive using the large swim-through to get to the other side when currents are in your favor. There is a buoy at the northwest end of the reef.
Near the sandy area, on platforms and in the large canyon spot for resting Leopard sharks, White Tip Sharks, Black Tip Sharks and spotted garden eels. In mid-water, just look out for schools of trevally, barracuda and tuna.
At the boulders around Shark Fin Reef a diverse marine life with large cube boxfish, clown triggerfish, half-moon triggerfish, and schools of batfish, palette surgeonfish, blue-faced angelfish and bannerfish can be admired. Some of them are always in disguise like the camouflaged devil scorpionfish. As the name suggests, scorpionfish have a type of “sting” in the form of sharp spines coated with venomous mucus. They are bottom-dwellers that feed on crustaceans and smaller fish.
Look out for smaller marine life in among the rubble on the sand between the coral patches. Leaf fish are seen here as are blue ribbon eels. Nudibranchs are numerous too.
Shark Fin Reef is also one of the few areas in the Similans to see Napoleon wrasse and groups of humphead parrotfish. Napoleon wrasse or humphead wrasse are beautiful coloured giants. It has thick, fleshy lips, and a hump forms on its head above the eyes, becoming more prominent as the fish ages, hence its name. Males range from a bright electric blue to pale green, a purplish blue, or a relatively dull blue/green. Juveniles and females are red-orange above, and red-orange to white below. Some males grow very large, reaching a length of 2 meters and can live for around 45-50 years. Humphead wrasses are hermaphrodites with some members of the population becoming male at about 9 years old. The species is most often observed in solitary male-female pairs.