This is the “cave” entrance to a “lagoon”, though the lagoon isn’t quite totally encompassed by the island.  This is a beautiful few minutes in a small boat, captained and crewed by kids probably in their lower teen years.  They have a blast, tooling around the tourist boats picking up fares in boats barely running.  Then they take them through the caves, into and through the lagoons, and back to the tour boats.  For this they actually get a pretty good fare and I’d expect this is a “good” job in this part of the world.