According to my book on nude beaches worldwide, I offer the following info.
St John - Solomon Beach. Within National Park. 15-20 min walk from Cruz Bay.
Go to National Park Visitors Center which is just north of the main hub of Cruz Bay. Follow the Lind Point Trail which starts behind the visitors center and hike along the hillside until you get to an unmarked but well worn fork in the trail, this fork decends to Solomon Beach. No facilities or services on the beach. Rangers generally leave nude bathers alone.
St Thomas - Little Magan's Beach. The beach is northwest (to the right when facing the water) of Magen's Bay. Walk 15 mins along the waters edge from Magens Bay then go around the rocks to the small beach that is traditionally nude and popular with gay men. Caution: there are reports of intolerance of nudity on St Thomas, so inquire locally about the current level of tolerance and be cautious if you decide to go.
To bad you will not be in St Maarten, there is a wonderful nude beach there that my partner and I visited in February, not too busy, and gay popular.
Have fun and if you go, please advise on these two beaches...