We were on the BI about 10 years ago and I was very interested in snorkeling, but also apprehensive. My daughter says I swim "like a mother" i.e. don't like to put my head underwater. The hotel we were staying at sold snorkeling equipment and I decided to ask them for advice. They sold me a mask, fins, etc. and the salesperson was incredibly helpful - he suggested I go and stand about chest-deep in the water and try it out. If I wasn't comfortable, he would come and help (they were right on the beach) and if I really couldn't handle it, they'd take the equipment back. It took me about a minute and a half to realize that I loved it and it wasn't any kind of challenge - in fact with the mask on I was very comfortable putting my head in the water.
A few years later I was on the Great Barrier Reef with a friend who was similarly apprehensive -she's not a swimmer, really - and she would only try it wearing a life jacket. (Didn't realize there was such a thing as a snorkeling vest at the time.) We paddled out and as we were both lying on the surface of the water admiring the fish, she realized that in that salt water I could stretch out next to her and not sink any deeper than she was, despite not wearing a life jacket.