Sorry, Sandylynn, I didn't see your post here. If you've not been to Grand Cayman before, Stingray City is the major attraction. Although, there is some really good snorkeling right near the tender area. As Dwayne mentioned, Abanks used to let you go in the water from their dock, not sure if you have to rent equipment from them or not. Belize is awesome for snorkeling. The reef is second only to the great barrier reef in size. You snorkel on the same reef in Roatan. We did our excursion through the ship. We went to Turnereife (sp?) and then they took us to a private island (Bannister I believe). It was a fantastic excursion. In Roatan, we did an excursion through the ship (Costa) the first time we were there that was great. The second time we snorkeled in Mahogany Bay. The last time we went to Tabyana. We were going to snorkel, but the weather wasn't very nice. The waves were up too high to make a beach snorkel enjoyable. Tabyana is a nice beach area, and there's probably pretty good snorkeling if you run into good weather. If you dock at Mahogany Bay and want to save some money, you can do a snorkel from the beach there. Granted it's not as good as being on an excursion, but it can allow you to put more towards another excursion in Belize. I think our excursion was $129/pp in Belize, something like that.
Hope this helps.