We were there a year ago on the Jubilee out of Galveston. You get on a bus at the pier and it takes you to a boat where you take a short ride over to the island. Once you arrive there is a buffet lunch prepared. Seafood, Mexican food - wonderful guacamole, lots to eat and drink. After lunch you are free to lounge on the beautiful beach, swim, play volleyball, or relax in a hammock. Really laid back. No organized activity except the lunch. After about 3 hours you catch the boat back to the bus and then back to the ship. The bus did let us out in downtown Cozumel so we could shop then we caught a cab back to the ship. Worked out great.
Here's a link that talks about it. I also read a critique that was not too favorable about the hygiene, something about the washing plastic used glasses in murky white water and reusing them, and dirty restrooms. There are some nice pictures on webshots, also.