Getting there:
At the ferry dock in Playa Del Carmen, there are a few vendors/ferries that go over to Cozumel. They're all pretty much the same, so go with whichever is leaving next.
The ferry can make you a bit seasick, depending on the waves and wind, but there are snacks and drinks on most ferries, and I found that going outside helped a lot! The ferries are pretty upscale, with cloth seats and televisions.
Once in Cozumel, you'll right away notice how commercial it is. There are tons of shops, hotels, and restaurants. In this regard, I much prefer Playa Del Carmen. If you're interested in snorkeling, hail a cab right off the dock. Ask to be taken to Dzul Ha Beach Club, a bit north of the famous Chankannab Park and much less crowded. We were the only ones snorkeling there that day, and it was high season. Dzul Ha had lockers and a bar where you could buy sodas and ice pops. Afterwards, we jumped into a shallow pool to wash off the salt and get dressed by the rental locker they rented us for the day. You can jump right into the water off the stone steps, and within a few yards, you feel as though you're in a HUGE aquarium.
Every place is different, and I don't know much about renting, but if you're worried about it, rent from a dive shop in Playa Del Carmen before getting there.
In the water, you'll see barracudas, those cute little captain fish, sea urchins, gorgeous angelfish, colorful and docile parrotfish, all kinds of coral, lobsters, etc. The current pushes you south, so make sure you're aware of your surroundings every so often. Start off snorkeling up the coast so that when you're tired, you can float back to where you started. The fish hang in the water, slowing swimming against the current. This is a great opportunity to take photos underwater. Unlike the estuaries and lagoons, the water is quite clear here, and you'll see a lot more critters and fish since you're essentially in the ocean. When you're done for the day, ask someone at the beach club, and they'll call a cab for you.
This is possibly the best snorkeling I've ever experienced, and I would highly recommend making the trip out here.