What are the most worth wild water excursions to do at St. Thomas?

We are going to St. Thomas next mth. First time cruise. We are thinking of doing the snuba trip, the Boss under water scooter thing and a few others. I am reading that some of these excursions have very short underwater time. Any input that you might have would be greatly appreciated. We are looking to do an ex. with a lengthy time actually snorkeling, with the best fish etc. to see.