Hey, I was there a few years ago and it was incredible.
The snorkelling there is fantastic. I recommend getting both of the following maps:
Cozumel map Cozumel dive and snorkel map guide
Those maps are awesome, so helpfull.
Other than that, do some exploring along the shores near the hotel, you'll surely find a few spots that have good snorkelling right off the beach. Chaakanaab park is pretty good, the snorkelling can be corowded, but it's really easy and some pretty good sights.
I'd suggest renting a car and driving the East shore of the island (I think East... the lesser inhabited side). You can cruise along the shore and stop at all the little beach huts along the way. Excellent spots to stop and have a cerveza. (sp?) Also, the nature reserve at the southern point is pretty neat.
Finally, I'd suggest booking a tour to chichen itza on the mainland. It's a spectacular ruin, and you can get a day-trip that'll take you right from the cozumel ferry to the ruins and back.
Um.. I think that's it... LOL
No matter what, you'll have a great time!
Cheers!
Steve