Dzul-ha is one of the better shore snorkeling spots on Cozumel. It's about 10-15 minutes south of San Miguel by taxi. It is near the Club del Sol between the El Presidente and Chankanaab Park. Taxis are plentiful and you should have no problem returning to town. All the drivers will be familiar with it, but if not, just ask for the Club del Sol. Should cost about $5.00
There's a beach bar there and I'm pretty sure they rent lockers and snorkel equipment, but they may not have a lot available so if you can borrow from your ship that might be better. The folks are friendly and you can get snacks and drinks here.
The area doesn't look like much, but the snorkeling is good. Enjoy!