Saint Maartin is beautiful, as is Saint Thomas (I loved Meagan's Bay), but I don't think they have the places for snorkeling and outside activities like I think Playa does. Playa is about 40 miles south of Cancun, and draws more of the Europeans than Cancun does. It also isn't nearly as commercial. It also is a short ferry across from Cozumel. If you want more info, I can direct you to a pretty good site, where a friend rates many of the hotels around the Carribean.
Cayman Island seemed expensive. It's more of a place for all the cruise ship trade, and they seemed to cater to them. Although, playing with the stingrays is a once in a lifetime opportunity.
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