Most of the diving sites in Cozumel are too deep for a snorkel (usually limited to 20ft with a snorkel).

ANY TIME OF YEAR IS GREAT in Cozumel!!

If you want to scuba, you need to be certified as a PADI diver before you go.

If you want to travel and scuba, buy your own fins, mask, snorkel, gloves, wetsuit, BC.
Then it is much cheaper to dive with a professional company organizing outings.

Cozumel is very competitive for scuba diving and you should be able to get a very good price if all they need to do is rent you an air tank and get you out to the dive site.

ANY WHERE IN THE WORLD: You can hire a boat and rent a tank of air yourself and save a lot of $$$.

And often, if you hang around in bars and meet people on the beach, you can get a free ride on a scuba outing with a friendly boat owner, who will let you use his air tanks ;-) ;-)