Check out Chankanaab Park and the Dolphin experiences. We did the Royal Swim with Dolphins and it was unforgettable.
YouTube - Chankanaab National Marine Park in Isla Cozumel
We booked direct with Dolphindiscovery.com
Cozumelinsider.com has a lot of ideas for excursions too.
When in Cozumel we try to always go to Pancho's Backyard for lunch.