Hi Sue,
WE also did "BOB" in St. Thomas, and it takes a little while to get used to steering and things appear real close, but they really aren't. They have divers down with you, so if you have any problems, they are right there. They will put you into groups and you take turns going on a bob, while waiting, they will provide snorkeling equipment and you can do that or just relax on the boat and talk to the crew, they have some interesting stories.
Have fun!