St. Maarten has some beautiful beaches but be careful if you have children with you or you are sensitive, as some of them are topless and/or nude beaches. When we were there it was mostly cruise ship tourists and generally everyone was clothed but there were one or two that weren't.
What ship are you going on? I did the same itinerary in January. We snorkeled at St. Thomas which was great. There is also something called a "BOB" which is like a little underwater moped that people seemed to really enjoy.
Oh, and be sure to buy some Sheridan's (chocolate/coffee and vanilla liqueur) while you are there! You can't get it in the U.S. and it is WONDERFUL stuff!