I liked Magen's Bay, but don't plan on going back any time soon. I'll choose Lime Tree Beach or one of the many other small beaches on the island. All are public--just ask a taxi driver. That's how we found Lime Tree Beach a few years ago.
If even the Caribbean's mild waves are too much for you, you'll like the calm water on Magen's Bay. We took advice from crew on the ship to go early in the day before it gets crowded. We caught a taxi back to the port around 2:00 or 2:30. It was definately more crowded when we left, but not too too bad.
I guess if you never plan to make it back to St. Thomas ever again, I'd say go ahead and go and enjoy at least for a little while. If nothing else, you'll be able to say, "Yes, I've been to Magen's Bay"