I've been to Cozumel a number of times and snorkeled there about 4 times. To be honest, the first three times I snorkeled there I went on the west side of the Island (the side the ships dock on) and it was just alright. Nothing special. This past trip (on the Magic) we went to the East side of the Island and the snorkeling was phenomenal! The reef was pristine, in about 30 ft of water, but the reef itself came up to the surface. There were shallow tunnels and caves and nicks and crannies to swim into. The aquatic life was great as well. It was a bit of a swim out to the reef, but when we got out there it was all worth it. If I were you, and you want to do some great snorkeling, do it in Grand Cayman, or the East side of Coz.
Mrsfuzzmo