You can book the tours right on the ship. You will get a shore excursion request when you get on board. My favorite was the Myan Ruins/Xel-Ha. The Ruins tour is beautiful but really fast.....Xel-Ha is a great place for snorkeling, swimming w/dolphins etc. You can schedule those when you get to Xel-Ha.
I did "Mayan Ruins at Tulum" while in Cozumel. Booked through the ship. It was unforgettable but very long. Glad I did it, but this time, I'll be spending more time on the beach, etc. There's so much to do, you'll just have to go back.
If you go to the Carnival web site and pull up Shore Tours and click on Cozumel, all of the available tours will be listed. There will be a brief description of each and how much time it will take. The prices are not listed, but if you call Carnival direct they will tell you. We did Tulum and were very impressed with the tour.