I just did the same cruise. St. Maarten doesn't have much to see in the water but it's not terrible. I enjoyed it. However, from St. Thomas you can take a ferry boat to St. John which is far better than both. It is easy to make all of your arrangements very quickly at the island. Apparently there are two good beaches with good reefs. I don't know the name of the one I went to. Bring your own snorkel gear. Rentals aren't that good and often run out.