There is a place in St.Lucia that has this underwater arch that I fun to dive under. This is the first time I went under it. It's a lot easier than it seems. You have to dive down about five or so feet, then turn 90 degrees so that you are now facing under the arch, than go full speed (if you have not chickened out by then) under it and then go rocketing towards the surface. I never got a chance to do this, but you can also go under it while upside down and see the arch above you.
-- By Rigel