SXM has a fairly relaxed attitude towards beach nudity and is one of the leading Caribbean destinations for naturists/nudists thanks to Club Orient on Orient Beach.
Topless sunbathing is the norm on all the beaches on the French side, as in Europe.

As for full nudity, the only official clothing optional beach is the Club Orient section of Orient Beach. It therefore goes that this is where you find the biggest concentration of naturists. Nudity used to be allowed at many other spots on the beach, but this has recently changed, so it’s best to keep nudity to the Club Orient section, to the small, designated nude area at Coco's and to the more deserted northern part of the beach between Coco's and Palm Beach. It is also acceptable to walk the whole length of the beach in your birthday suit in the early mornings (before 8am).

Nudity is tolerated (although officially discouraged) on many of the other beaches on the French side. However as the other beaches are not true nudist beaches the rule is to be aware of other people, families with children and people’s homes. So don’t just walk around or sit naked wherever you please, but choose quiet areas, where you’re not in anyone’s way. If people show any sign of annoyance, move to a more secluded area of the beach, there’s always a quiet spot. There’s talk that gendarmes (the French side police) have been turning up on some beaches recently asking naked people to put their clothes back on, so, unless you are at Club Orient it is best to get dressed if some official-looking people arrive… and if you don’t see them coming, don’t panic, just respond politely and cover yourself if they ask you to.

You will find the sites like SXM Uncovered SintMartin.net and St-maarten.com useful in finding out more about deserted beaches where you can sunbathe and swim in the buff and also activities you can do on island completely naked!