Koh Pos is the small island just off the headland of Sihanoukville, joined to the mainland by a long road bridge. The island was earmarked for a major Chinese tourist development that stalled; most of the surface is empty and the headline resorts unfinished.
It is still worth a visit for the view back to the city skyline and for the small fishing village on the far side. The drive across the bridge is itself a Sihanoukville experience. The seafood restaurant at the foot of the bridge is reliably good.