If you are thinking about diving in the Caribbean, Roatan (an island off Honduras, sitting on the Belize barrier reef system) and Bonaire (Netherlands Antilles, off northern Venezuela) have been our two favorite spots. Bonaire has probably the healthiest reefs in the Caribbean, and it doesn't get huge cruise ships, so it has a quiet pace. Roatan is also slow-paced, and the reefs are excellent. However, snorkeling in Roatan is limited by the distance from shore to the reef, and there's little to do aside from diving in Bonaire (fine for us old fogies).
On the other hand, I always have a great time in Key West. The diving and snorkeling are very good (tons of fish, corals also, but not quite as pristine), the food is fantastic, and there's a night life if you want it. I can even recommend a nice, quiet little B&B that's right in the historic district.
Cozumel can be nice too. You can fly into the horror of Cancun, and catch a ferry over to Cozumel. It is still a tourist town, but on a much more manageable scale than Cancun. If you get tired of watersports, you can head to the mainland and see the Mayan ruins.