akumal has AMAZING snorkling, especially out by the reef, on the left near the opening the boats go in and out of. sharks, turtles, coral, the whole bit.
Dos Ojos Cenote is a MUST dive (good snorkle, too) they say it has "unlimited"vis, and i believe it, it seems you are literally floating in air, you can't see the water at all) there are many other good snorkle cenotes.
near akumal, there is a river/cenote (forget the name, but there are statues all over the property) that flows into the ocean, very cool to snorkle with freshwater fish, then out to saltwater fish/coral.
of, course Cozumel is great, be prepared to dive deep if going to some of the more famous spots (devils throat comes to mind, personally i think the dive above the reef is better than the dive "through the throat")
if you a very adventerous (like my wife and i) there are some REALLY good dive/snorkle spots from the shore, one has a big hole on the ground, about 30' deep where a freshwater cenote comes up in the ocean,,again really cool (DO NOT go in the underwater river unless you are VERY experienced with cave diving, not even to stick your head in and look around)
these can all be found with a little research (locogringo is a good one)
night dive is also a good option there...