I seem to remember a dive shop advertisement at one of the shops on the beach that would let you puddle around off their pier with an instructor for a short time, to see if you wanted to go for a resort course. Am I fabricating memories? Who knows. My brother-in-law, who just had a few years snorkel experience under his belt decided he wanted to try a resort course (he had spent a few years listening to the rest of the families dive experiences). By the time we got out to Hol Chan, and he'd spent about ten minutes in the water with the instructor, off we went. His wife would only let him go if I went along. I don't think he's ever had that much fun, took to it like a natural. The next day we went out to Mexico rocks with a snorkel trip, and he and I dove (the instructor was along, of course). Granted, the ten foot depth was no great shakes, but you would be amazed what you can see when you are down for an hour in a relatively small place.
I think that if you hit a dive shop, they will let you get in the pool for a short time to see if you can handle being down. If you can't, it will become very obvious very quickly. Being comfortable in the water is a large part of it.