Another vote for Belize - I did some snorkelling there and got fantastic pics with a rented Sea&Sea MX-10. There's (I think still there) a dive shop on Ambergris Caye called Mike's (I think - it's been over a decade since I was there) that rents underwater camera gear and gives really good advice. A really good intro dive trip is this american ex-pat who runs an old Chesapeake Bay oyster boat for day trips out to Caye Caulker and back - he does a stop at Shark-Ray Alley, where you can watch scuba divers feeding (and filming!) the nurse sharks and stingrays swimming around.
If you go to Belize, step off the islands and go inland to some of the Mayan sites - Lamanai is terrific, and is not so heavily touristed. Stay at Lamanai Outpost Lodge - it's an eco-resort right next to the site. They have their own guides who will take you on hikes through the ruins, nature walks, and even nighttime wildlife spotting tours on the lagoon.
If Belize is a budget-buster in terms of transportation to/from, go down to the Florida Keys. There's good diving/snorkelling off Key West and Islamorada. You can save yourself a bunch on the plane tickets by flying in to Ft. Lauderdale or Miami, and driving down. Both are pretty commercial, but there's lots of activities both on land and off.