One of the highlights of our last trip to Belize has to be the day we went snorkeling with the nurse sharks and string rays in "Shark Ray Alley" off the island of Ambergris Caye. Getting to Shark Ray Alley is only a short boat ride from the island and you won't have a problem finding a guide on the island to take you there.
Nurse Sharks are considered mainly non-aggressive but they have very powerful suckers rather than teeth, kind of like a catfish. If they latch on to you it can be bad.
It can feel a bit intimidating sitting on the side of a boat about to jump in with these sharks, but it is well worth it.