We have just returned from Playa Del Carmen and took friends to Cozumel for the day. Snorkeling can not be beat anywhere. If you snorkel in the morning, you can snorkel at Dzul Ha. It has a deck as opposed to a beach, but it has some of the best snorkeling on the island. Later in the day, the current seems to get a bit strong.
Morning is better.
I would eat at Dzul Ha as well. Their prices are reasonable and their food is good. Their drinks are icey cold. Use the afternoon to see the east side beaches. They have a personality all their own.