I second zimmyfan on the Akumal recommendation. Beautiful place with very high chance of seeing large turtles. I've heard that Half Moon Bay (just north of Akumal's main bay) has more turtles, but I didn't see any there. I saw several in Akumal's main bay. I later heard that you need to swim out to the north part of Half Moon Bay to find the turtles, which I did not do.
Cozumel has some good shore snorkeling at Playa Corona, which is not far south from the main town on Cozumel. A taxi will take you there for around $5 I think. There's a bar/restaurant there where you can order some lunch and beer, then head into the water. I saw some French Angels and a large Queen Trigger there.