How much time do you have? I don't know that you have enough time to do everything mentioned. But you could rent a vehicle and drive the loop aound the island. The ruins on Cozumel are interesting but less than remarkable. If you have your own snorkling gear, you could stop any where along the south and western side of the island, San Francisco beach has a reasonable reef and isn't too far from the International Pier. The eastern side of the island has rough surf, but some nice beaches, not really snorkling water. Cozumel is not really 4x4 country, you could rent a cheaper vehicle, like a VW and drive to Chankanaab Lagoon on the south end of the island.
I guess what I am suggestting is that you figure out what you would most like to do and plan from there. You'll need to split up if you're going to try and do everything on the list.