Visit Anse du Gouverneur. Swimmers here must contend with a stubborn undertow, so time your interludes in the surf with the current.
Consider Anse de Grande Saline, a large beach where you can feel secure in your privacy, even in a crowd. At the end of the beach, you'll see nude snorkelers. There are no facilities here or in Anse du Gouverneur.
If you are looking for a beach with amenities you can visit Baie de St. Jean. Although it is not clothing optional, you can sunbathe topless on this beach.