I go to St. Martin every year and visit Orient Beach. You will see nude people, don't stare at them it is considered rude. If you feel the need to take off your top, go ahead. The far end of the beach is were you will see alot more nude sun worshippers but that is probably because they are staying at that resort. I have seen completely naked men strolling the beach, completely tanned also and they don't seem to have a care in the world. It is a beautiful beach and there is alot to do there. You can parasail, jet ski, get a massage, each at one of the restaurants. It is usual crowded and you will have to pay for a chair and umbrella if you want to use one.
They have a less crowded beach called Friar's Bay which is where I go, calm waters, stilll have eating and drinking, chairs and umbrellas but the water is calm. Orient is a bit harder to get in and out of, more waves. But then again, I am older.
enjoy your trip.