Having been to Cozumel on many occasions by cruise ship as well as staying there for a couple of weeks on vacation, and being a very active scuba diver for the last 17 years, I can tell you that snorkeling in Cozumel is good, but not great. Fish like to stay around the coral reef - it provides food and shelter. Problem is, the really good reef doesn't start until around 45' deep. There is some reef closer to the surface, but compared to other places, it's disappointing.
On your itinerary, if you want to do some nice snorkeling, then I would suggest looking at Grand Cayman instead. The reef is closer to the surface and you can see more fish. Plus, you can do it right near the tender pier and if you know where to go and have your own equipment, it's free!