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. Get certified now!

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.
You can hire a boat and rent a tank of air yourself and save a lot of $$$.