There is a wall on Compass Cay (Bahamas Islands; Exuma Cays) with underwater entrances to two large caverns or grottos. At low tide, you can swim in a small crevice with your head above water. At higher tides, it takes a shallow dive into the wide entrance and up into the cavern. Spectacular places.

Here, we are underwater in the grotto with the opening from blue glow open water behind.

Model: Kristi Sherk (WashDC area)
Tail: Eric Ducharme' Koi Tail (themertailor.com)