If you only have a half day, maybe it's best to take an excursion. I'm not familiar with St Thomas, so I can't say for sure. I would normally tell you to get to St John, but a half day might be too short for that. My experience on a Caribbean cruise is that every single port was nearly identical in the "tourism" aspect. Every place seemed to sell the same crap and we rarely found a beautiful, quiet beach. That being said, if you only have a half day, I think it might be wise to see which excursions are available and take the opportunity to get out of the touristy area of St Thomas. You'll still be with a bunch of people from the cruise, but maybe you'll get to explore a great snorkel site or a less-populated beach.

I'd opt for a few hours at Magens Bay given your short stay. The tour or taxi trip there will afford you a bit of a glimpse of the island and if you're looking for some P & Q away from the cruise ship people, head LEFT down the beach with your towels and a small cooler.

There is a concession restaurant at Magen's but the cruise lines will probably put together a couple of sandwiches for you if you prefer to be a little more adventurous. Not much in the way of snorkeling adventures at Magen's but if you do have masks and fins in tow, the far left around the rocks is the best spot.

Then again, if you're into shopping then there are many options... Enjoy your stay here!