Mike I just returned from St. Thomas a week ago and although I was scuba diving there the SNUBA-ers were right along side us. Basically the SNUBA folks have the tanks floating on the surface with a long regulator tube goign down to them. Up to four folks can use the float. This setup frees you from the weight of the gear on your back and offers more bottom time then snorkling. Most of the 3 hour trip is spent driving, boating, learning. You have around a 1hr bottom time, DEPENDANT ON AIR SUPPLY, this is something noone talks about. If there are only 1 or 2 folks on the bottle then it last around 1hr 4 folks on the max of 2 tanks will give around 45minutes. There was a group that in their excitment of the georgous underwater fauna ran through the tank in under 30 minutes.
As for what you will see... many species of fish and coral (yes coral have species as they are living organisms) chances are there to see baracuda, stingrays, turtles and if you look close many flounder. They guides are all PADI certified and very well versed on their habitat.
Go and enjoy. Dont worry about all the details, so many they can get confusing. Go enjoy seeing the underwater world that few folks get to enjoy.