I am pretty relaxed about these kinds of things and I would still suggest caution. When we were there I first went out with my girlfriend. She can swim ok but gets nervous in open water. She did fine, but found it pretty tiring (it is a long swim - about half an hour out and half an hour back).
Later I took her husband and my husband out. Our friend does not know how to swim but has been practicing floating. He is reasonably confident and not the type to panic. With the life vest (inflated), snorkel, and fins, he felt comfortable enough to attempt it. My DH can swim but does not care for long distances and while physically fit, is not a strong swimmer.
Our friend did well, liking his fins and feeling comfortable in the inflated vest. He swam pretty well with all his gear. My DH likewise did ok, hanging on to our friend or to me sometimes for a rest - I insisted on several floating breaks.
They both survived the snorkel trip but both found the trip back very tiring. I knew I would get them both back but it took quite a while. The main reason they were fine is because both are calm individuals not given to panicking. When tired, they had the sense to take breaks and float in their inflated life vests. They both enjoyed the snorkelling and were happy they did it.
As for me, the snorkel was fun...but man, I sure enjoyed the lengthy swim! I usually swim laps..so to have the freedom to swim and swim and swim in a safe place and only do ONE lap makes
Castaway Cay my very favourite place in the world to swim. My DH always reminds me to be cautious and swim reasonably close to people and shore when I swim outdoors. Which generally means I Am still stuck with swimming laps both in lakes and in the sea.