Chankanaab isn't that close to the ferry, but then none of the beaches where you can snorkel are - they will all be at least a 10 min taxi ride.
When you arrive by ferry you will be offered snorkel tours by vendors on and around the ferry pier. These could be snorkeling or glass-bottom boats. Most of these are trustworthy local operators and if you see one you like the look of and negotiate a good price you could go with them. They may not take you far from the ferry though, so the snorkeling might not be the best Cozumel has to offer.
If you're serious about your snorkeling I recommend you take a boat tour to the reefs at the south of the island. There is a 2 hr tours that leaves daily from Paradise Beach for example.
Cozumel Insider mentioned above offers information and bookings for tours. You can also try www.thisiscozumel.com/tours - they offer the Paradise Beach tour and some others.