Twin Peaks at Ko Tachai is one of the best dive sites on the Similans, although strong currents mean it is generally reserved for experienced divers. Expect to see big fish: manta rays and leopard sharks, as well as turtles. Ko Tachai is periodically closed to day trippers in response to growing environmental concerns; dive boats can still do dives here while remaining offshore.
Ko Tachai, along with Ko Bon, became part of the Similan Islands Marine National Park in 1998, when the park was expanded from its original nine islands.