All beaches on the French side of St. Marten are topless. Orient is the only official totally nude beach option. St. Thomas nude beaches are all unofficial. Fred is right ask a cabbie.
You can get a tour from your ship to go to Orient Beach on St.Martin. We've been there two or three times now; it is a very beautiful beach and the lunch at the french restaurant there (which was included in the price of our tour) was one of the best luncheons I've ever had. They actually call it "clothing optional" beach, and kids and whole families are there. If you walk down the beach there is a totally nude beach, where you can walk down and back if you'd like but there is a big sign there saying no cameras please. My husband and I enjoy going there, but we don't take all our clothes off, we leave that for the young; but what I noticed the most there, was older men walking up and down the beach in their birthday suits. Enjoy!!!!!