Hi MaureenNC, I really wouldn't waste my money in St. Maarten. We've done 3 snorkeling trips there and there wasn't much to see. The water is clear, but there wasn't a great deal of colorful sea life, coral was kind of non-existant.
If you're up for a little traveling, when you get to St. Thomas, take the ferry to St. John and go to Trunk Bay. There is a self-guided snorkel trail about 50 yards from the beach, normally, if there's not a front moving through, the water is like glass and crystal clear. It's awesome, one of the top 10 beaches in the world, it's also a marine park. There are some great catamaran/sail excursions to Buck Island as well. Sapphire Beach on St. Thomas is good for snorkeling, IF the wind isn't blowing too hard. There are lots of rocks getting into the water, the north end is the best entrance, but there's some good coral and fish there. Megan's and Coki are good. Of the two, Coki probably has the better snorkeling, but it can get very choppy there.
It's been ages since I snorkled in Nassau, too. But just about everywhere in the Bahamas is good snorkeling. Coral reefs seem to abound in the Bahamas. Normally, I recommend going to the Atlantis (take a cab over) and after touring Atlantis, go to Cabbage Beach, which is adjacent to the property. It's a free beach. I don't remember what the snorkeling was like there. Last July when I was there, we just toured Nassau a bit and I didn't get to snorkel.
Hope this helps a little.