Losin is a granite pinacle about 150 kilometers due east of Songkhla in southern Thailand. There's a light (non-functioning) on the rock, which only breaks the surface at low tide. The pinnacle is relatively narrow towards the surface, but about five meters down it broadens out into large areas of nearly flat surfaces.
The rocks are covered in places with hard corals, and there are large swaths of staghorn coral on the landward side. However, there are still big areas of bare rock. This is probably due to the abundance of parrotfish in the area, which can be seen grooming the rocks. Several varieties of parrotfish can be seen, including very large bumphead parrotfish.
'Large' is the order of the day at Losin - big schools of fish, and large examples of many common reef fish. I saw several pairs of huge Blue-Ringed Angelfish and it seemed like mature couples of Moorish Idols were everywhere. The big schools of fish were yellow-tailed Trevally and others, while up near the surface there are almost always baracuda on the lookout for their next meal.
Losin is only reachable by live-aboard dive boats. Only those boats which shift from the Andaman to the Gulf during off season for the Similans visit Losin, and even then only when they're shifting one way or the other. This means there are only two chances a year to visit Losin, so very few divers get to see this place. One of the few dive boats that still makes this transition is the Scuba Explorer.