To us it appears that snorkeling at St. Maarten is iffy at best. They get a lot of wind that kicks up the sand and reduces visability.
DCL does offer several snorkeling excursions. If the conditions are bad for snorkeling they will sometimes let you out of the excursion. There is the golden eagle catamaran that does the sailiing, snorkeling, swimming and beach party. It's a lot of fun and if the conditions are right you can get some good snorkeling in.
One of the excursions I'm interested in is the shipwreck snorkel, but it appears that no one has done it because no one has ever commented on it - but it does sound interesting.
There are several nice beaches on St. Maarten that you can go to on your own. The one at the port/in town is ok. Orient Beach is breathtakingly beautiful. If you are concerned about nudity or ladies walking around topless, ask to be droped off on the "family" beach (much less nudity there - and there really isn't that much nudity happening there anyway). We thought that about 95% of the people were "appropriatly" attired.
You say you are going to the beach on St. Thomas. There are many great snorkeling trips there. Many near St. Thomas / Buck Island and many over at St. Johns (Trunk bay, barefoot catamaran, 5 star, etc). Maybe you should rethink your strategy/agenda to plan on snorkeling here and doing your beach day on St. Maarten.