Shelling around Ambergris Caye can be great. Here's a list of places you can go to find a variety of nice shells:
1. any one of the recent dredge sites south of town on the back side of the caye (or if they're running, along the front side of the caye). You can get there by golf cart or boat, and the piles of sand that are being dredged up are readily visible. You'll find all sorts of shells there, including snals (gastropods), clams (pelecypods), and some corals. Be on the lookout for Maya artifacts (fragments of pottery and chert tools) and conch shells opened by the Maya (they'll have round holes punched into them instead of elongate holes).
2. Immediately north of "The River" where some sort of development is being built; the area is reclaimed swamp filled in with dredged material. Lots of shells there, as well as Maya artifacts and conch shells.
3. up north at Robles Point is a huge pile of corals (dead) and snails, sea fans, sponges, sea urchins, etc that have been washed up over the years. Also walk the beach north of the the rubble pile (which is obvious when you get there) and you're likely to find many beautiful specimens
If I can ever get off my duff, I'll publish the shell book I've written for shells in northern Belize. It identifies every one of the shells found in the area as to common name and scientific name, where they live, and how they "make a living". Good luck, bring lots of collecting bags, and have fun.