We were there in January. In St. Maarten we took a cruise aboard the Schooner Lord Schffield. Fantastic excursion!! They sail down to the other end of the island where they drop anchor for you to swim and snorkel. They serve BBQ Ribs and Chicken, all the drinks you want (altough it doesn't get rowdy like the party boats do) Also smoked oysters, french bread and cheese and its all included in the price. Here's a link to their website.
http://lordsheffield.com/
In St. Thomas we took the ferry over to St. John and then a taxi to Hawk's Nest Beach. It is a park so no bars, shops etc... but very uncrowded and you can snorkel the reef right off the beach. The ferry over to St John costs about $4 each way per person.