Leinster Bay
Leinster Bay is located on the north shore of St. John. It is largely surrounded by rocks, but there is a small sand beach that is a great base of operations for a day of relaxing or snorkeling. In order to get to this beach, you will need to walk along a trail for 15-25 minutes (depending on how fast you walk), or get there by boat.
Because of the small beach and overall rocky terrain, Leinster Bay is not the ideal location for someone that just wants to lounge on shore. It will probably be deserted, so at least there will be privacy, but there won't be any long walks down the beach.
However, for those interested in snorkeling, this is reported to be one of the best locations around St. John. Barracuda, rays, turtles, sea horses, starfish, and of course coral and many species of fish have been reported. Also, Watermelon Cay is nearby, so the more adventurous could swim across and enjoy the amazing underwater sights that it boasts.