If you want to stay on St. Thomas, Coki Beach, Meagan's Bay, or Sapphire Beach are nice places to spend the day. If the weather is nice, go to St. John. You can grab the ferry from Red Hook or from Charolette Amalie. Then take a cab to Trunk Bay, it is one of the top 10 beaches in the world. There is a self-guided snorkel trail about 50 yards off the beach. There are lockers, restrooms, showers, a couple of shops, and a restaurant. It is absolutely beautiful.
In St. Maarten, you can go to Maho Beach (where the jets land on top of you), Orient Beach (nude beach possibility), or there's a beach near the port that you can take a water taxi to. The reason I mention nude beach possibility is that there is a section of Orient that is clothing optional. Unfortunately, the ones that choose to not wear clothing, shouldn't and they don't stay in their area. They walk the beach.
The last time we were there, we did the Tiki Hut snorkel excursion. It was a fantastic way to spend the day, snorkeling was good for St. Maarten, but the operators of the tour were what made it. There was an area for those who did not know how to snorkel, beach chairs and water chairs, a bar/restaurant, and just really nice people. We did it through the ship, but you might be able to book on-line at snorkelstmaarten.com. The operators will take you either back to the ship or back to town for shopping. There also is no time limit, you can stay as long as you want.
Hope this helps some.