Divers will enter the dive site at either the north end (right next to the southeastern tip of Sharkfin Reef) where the mooring is attached to the reef at 20 meters, or at the deeper south end. This decision will depend on the currents, which can be strong at Boulder City. Some divers will use the line to get all the way down to the bottom then use the rocks and boulders for shelter as well as exploring for all kinds of marine life. There isn’t a great deal of coral at Boulder City, but this doesn’t reduce the quantity or quality of fish and invertebrates there. The massive granite boulders are spread out in an unnaturally neat way, making the dive site appear to be a huge loaf of sliced bread, with large rocks on the sandy seabed being the crumbs. Visibility is normally excellent, especially during the stronger currents, so if something large swims by, there’s a better chance of seeing it. Manta Rays and Whale Sharks pay visits from time to time, especially late in the diving season, but are by no means guaranteed. Reef sharks and smaller rays can usually be found out on the sand which surrounds the rocky dive site.
The lack of coral is not a problem, because in its place is a vast covering of algae, attracting grazing fish and invertebrates and, consequently, those who prey on them. Some of the larger bony fish to be found both here and at Sharkfin Reef are Humphead/Napoleon Wrasse and Humphead Parrotfish, both of which are slow, steady and solitary swimmers. Larger fish in schools include Barracuda and Snapper Butterflyfish Angelfish , and Unicorn Fish are among the more colourful and medium-sized reef fish grazing on the algae here. Such good visibility is not just an advantage for seeing fish far away. Those who enjoy underwater photography can benefit from getting the most amount of light, and therefore colour when snapping away at the colourful fish and nudibranchs which can be found all over the granite rocks and boulders. Sheltering under the deeper areas, Oriental Sweetlips are colourful and don’t appear to be afraid of being approached. Giant Moray Eels are the most popular of their type, but other more colourful species are there, too. For those who know what to look for and identify, it’s possible to see a species which is endemic to the Similan Islands. The Andaman Jawfish is neither exciting nor attractive, but it’s new to science and only lives here.