I saw a shark in Cozumel. He was not very big (2 ft?) and very well camoflaged, just sitting on the coral. It was really neat and not scary at all. The dive guy had to swim down and get him to move or we never would have noticed him.
This is not on your itinerary, but we will be taking our kids to the Blue Lagoon in Nassau in a few weeks. There is a nice protected lagoon where I plan to teach my DD 11 and DS 6 how to snorkel. It should be a really good starting place.