The best place to snorkel is the Chankanaab Park, check out their web site. I was there last year and was amazed at all the fish they had. I was unreal, especially with all those people there. Take a cab and go yourself, or rent a car, snorkel and then explore the rest of the island once you tire of this place.
The other side of the island has the most beautiful beaches. Also if you get over there, stop at Punta Morena and get some lobster while you watch the surfers. What a relaxing enviroment and it is cheap...beer lobster, just watch the amount of sun you get.