Ko Similan is the largest island. The sea in the area has an average depth of 60 feet. Underwater it is full of rock formations and coral reefs in several shapes and forms, resembling such things as deer, leaves, brains, and mushrooms. Similan Islands are located just 84 km northwest of the bustling crowds of Phuket. Koh Similan is one of the best-known island groups in the Andaman Sea, largely because of the wonders that wait beneath the clear blue waters that surround it.