Boulder City- Only 1 kilometre southeast of Similan Island 3 (Koh Pa-Yan), and about 400 meters of Shark Fin Reef, hundreds of huge granite boulders and rocks are spread out underwater in such a way, giving the impression of a city layout. Boulder City is a deep and expansive dive site where the shallowest of boulder lie around 12m and the deepest at 40m with gaps of 30-40m between the 3 main pinnacles, stretched out on a sand bottom.
The boulders are generally smooth with few corals although the surface area of the boulders is covered with soft coral like the stunning fluorescent pink Anemone Coral. Golden gorgonian fans and sea whips feed in the currents at the sides and in the cracks of the boulders.
Boulder City is a great searching site, looking in crevices and finding colourful marine life.
All about the dive at Boulder City -Similan Islands
Boulder City is an exposed, deep dive site and currents can be strong but for this reason it attracts bigger fish such as whale sharks and manta rays, especially late in the diving season. The dive starts deep and gets gradually shallower as you work your way in and around the rocks in an S-pattern route. Where to descend depend on the currents but on different sides on the site mooring lines will help you to descend and ascend which is recommended. The depth of the sand bottom at 35-40 meters is generally too deep to follow the whole dive. Instead try to stay close to or behind rocks and keep an eye on your decompression time.
Visibility is normally excellent, especially during the stronger currents, so if something large swims by, there's a better chance of seeing it! Reef sharks and smaller rays can usually be found out on the sand which surrounds the rocky dive site. In the deeper sections, there are many red and white whip corals. Here you are likely to spot small schools of Oriental sweetlips, blue-lined snapper, barracudas, grouper, triggerfishes, parrotfish and filefish. Other sights to look out for include Napoleon wrasse, turtles, and trevallies hunting swirling schools of glassfish.
The abundance of algae growth here attracts plenty of grazers, including nudibranchs.
Nudibrachs are one of the most colourful creatures on earth and some divers are explicitly passionate about them. To avoid their predators some species can mimic their surroundings while others have intensely bright and contrasting colours to make them seem suspicious or poisonous. They vary in adult size from 4 to 600 mm. Nudibranchs are often mistakenly called sea slugs.
There are masses of unicornfish, butterflyfish and angelfish happily feeding away. Sea urchins can be found wedged into the cracks, along with lionfish, scorpionfish and large moray eels. Scorpionfish always seem to hang around. Longfin batfish, black damsels and fusiliers are likely to accompany you on your safety stop.
Enough marine life to get underwater photography lovers really excited.