Koh Tachai lies about 20 kilometers north of Koh Bon, but is still part of the Similan National Park. The island has a nice long sandy beach on its southeastern coast, with only a park ranger station keeping it from being the perfect desert isle. The island makes a good overnight mooring spot for those making a run up to Richelieu Rock.
However, the island does have a couple of good dive sites of its own. The Tachai Pinnacle, off the southern tip of the island, is a fairly spectacular site in its own right. The reef off the east cost of the island is also a nice end-of-day dive spot.