I would go to Grand Cayman (and been a bunch of times), but they do not have all-inclusives on the island. Snorkelling it great, sting ray city would probably be enjoyed and is a family friendly island. Not crazy at all...you could rent a house/condo instead.

The Marriott has great snorkelling right off their beach, but of course rent a car and many other places abound to snorkel.

Cheers.