Speaking of SHARKS, you can swim with them in AC at Shark-Ray Alley. When the boat pulls up there the rays and sharks (nurse sharks) come from everywhere. It is awesome!
We had done it once before in Grand Cayman, but I like it better in AC. You have to do it at least once. My first snorkeling trip in AC (many years ago), I jumped from the boat and landed right on top of a nurse shark (didn't actually touch it but almost). Scared me to death but the shark just swam a little bit away and settled back.