We did "BOB" a few years ago in St. Thomas and thought it was wonderful, just a different kind of excursion. The actual time you are on the Bob is only about 20 minutes, but while you wait your turn or are finished and waiting for everyone else to go, you can relax on the boat, or they provide snorkeling equipment and you can snorkel right there too. The crew is wonderful and have interesting stories to tell. I would buy an underwater camera at home before you leave, they will be selling them once you start the excursion, but at twice the price you can buy it at home.
Have a wonderful time!