We did two things when we were in St Thomas - both of which I'd recommend.
One was the cable car which is within a 5 minute walk from the ship and gives you some wonderful views over the bay (great pictures to be had).
The second was the Buck Island Sail and Snorkel which we did in the afternoon - the nice thing about this is there is only 6 to a sail boat, so it's really pretty cool! As it's only a half day sail, it's better suited to first time snorkelers and not so long for you to sit out as the non-swimmer.
I personally don't recommend Highpocket. They are the same family/company as the Fantasy daysail. I booked an all day sail with them back in <b>February</b> for New Years Eve. At the beginning of December they told me completely out of the blue that they were moving our party from the Fantasy to another yacht in their fleet - Highpocket, a day later they then told me they made a mistake and Highpocket was already booked but they wished me luck booking another excursion thru our cruise ship. 2 weeks before I was due to leave - did I mention it was New Years Eve?
That was it...no explanation - in fact, there was not even an apology and despite me replying to them, they never even had the courtesy to even acknowledge my existence after that. I guess they were offered more money by someone else, I can only surmise that due to the fact that they never bothered to offer any sort of explanation for cancelling a booking I'd made over 10 months earlier - personally I wouldn't touch any company with those kind of work ethics and customer service (or lack of) with a barge pole, and I certainly won't be recommending them to anyone who asks!