Sorry, I didn't see this post earlier.
Grand Cayman; awesome beach at 7 Mile Beach. Try Calico Jack's - great beach, good food, cold beer, and lots of water activities. If you like to snorkel, then you have to try Stingray City!
Belize; scuba diving is awesome!
Cozumel; scuba diving is awesome! Not great snorkeling, but some nice beaches.