Which company are you using?
We used Cozumel Cruise Excursions and had a great time. We went all over the island. We went to the top of a lighthouse, went snokeling, saw crocodiles, saw some ruins, had lunch (included), shopped, and visited a tequila place that told the history and how it was made. It was a private tour so we had the option to change things if we wanted but we just kept the itinerary he suggested. He also had a cooler of beer and water for us.
I felt like we really got to see and do a lot.
Oh, and the snorkeling is at a private resort and you enter from the shore. There are chairs and a pool there as well. I did not snorkel (I am not great at it) but just waded out a bit and saw a lot of fish in the waist high, clear water. My friend who did snorkel said the guide was great and pointed out all kinds of interesting sea life.