We did the "BOB" underwater excursion when in St. Thomas and did think it was worth the money. Although the actual time on Bob is only about 20 minutes, its still very thrilling. Takes a little bit to get used to driving, things seemed so close that really were not that close, but truly a different excursion and we enjoyed it. While you are waiting your turn on bob, you can snorkel or just relax on the boat, some ot the crew were very intersting to talk to.
YOu really aren't down that far where your ears will pop, at least I didn't have that problem. They bring fish food and get tons of fish swimming around you and then take your pic with your camer. Be sure to purchase a water camera at home before you go, they will sell you one, but twice what you'll pay at home. There is a trail they take you through and they also have a diver there for every 2 bobs, so if you run into any kind of trouble, they are there to help.
Yep, been to Orient beach, lost to do and see there, parasailing, jet skis, etc. Tons of beach chairs and restaurants too. You can easily just grab a cab there, and there are plenty to take you back to the port.
Last trip here, we did the Rhino boats, what a gas, bet your son would get a kick out of those. We had to book that through the cruiseline, lots of fun!