My family was in St Thomas in March of 2002, and we had a wonderful cab driver, God Father...LOL. We found him when we were looking for a ride back to the ship for lunch and after we ate we asked for him again when we left and he took us to a scenic overlook on the way to Magens Bay. Once at the beach he arranged a time to pick us up and at the arranged time he took us back to the ship.
There was some traffic on the way back, and while sitting in it he told us all about St. Thomas. He was great.
I would definately reccomend Magens Bay. It didn't seem crowded when we were there, even though it was "spring break" season. Go down towards the end of the beach, it gets quieter and more secluded.