stevemac reminded me of something... Get an underwater camera or housing for your camera. Lots of fun!
Here is a quick tour around the beaches of St John. Starts at Cruz Bay and goes clockwise around the island. Well it is not all of them, but some them... I did this about 3 years ago and probably should update with more beaches and better pictures, but you will get a sense for what you are in for. Enjoy
The Beaches of St John