The Tulum Ruins are not in Cozumel. You would have to get on a ferry, then a bus and that will take you hours, especially if you go with a crowded tour. To get to know the island, there is a nice way of doing so - there's this tour that takes you around all of Cozumel. It's called the Dune Buggy Tour, I believe. The take you to a bunch of different spots, you shop and snorkel and get fed, drink all the beer you want and on the way back, if you make a special request, they will drop you off at San Gervacio, where there's an old Mayan village that has been very well preserved. It'll be up to you to get back to the cruise ship from there, there were taxis when we went. The ruins aren't as big as in Tulum, but you still get a good feel. Save Tulum for another vacation. It's hard to appreciate it all when you're tired from the trip there.