Here is part of my review I did on this forum -"back from triumph" - pertaining to our port visits including St. Martin.
Our stops were San Juan, St. Thomas and St. Martin. We didn't get off the ship at San Juan but in hindsight we should have - at least for a walk around the old town. Did the Champagne Catamaran Sailaway in St. Thomas. Snorkeled in St. John's Bay and enjoyed champagne, cheeses, fruit and bread on return trip. I highly recommend this but book early as space is limited. At St. Martin we took a cab ($7 each) to Orient Beach. We rented a chair and umbrella at bikini Beach - Orient Beach is divided into lots of beaches. We had lunch at the restaurant right at Bikini Beach - AWESOME!!! If you go to St. Martin do the same as we did and do it on your own - there are lots of taxis and you don't have to worry about getting back to the ship.