Tami, maybe I'm misunderstanding what you are saying. If you go to Carnival's home page, there's a shore tour section--click on that and then the port of Cozumel. It shows the tours available. It shows one to Tulum. You probably are aware that Tulum is on the mainland, approx. 12 -14 miles across the channel from Cozumel. What ship are you going own ? Maybe your particular port time wouldn't allow for this excursion.
The ferry ride is about 30 minutes or so and at times can be a little rough. Once you reach the mainland, there's a bus ride to Tulum-- I can't remember for sure how long the actual bus ride is but I'm pretty sure it's less than an hour. The tour will pretty much take up your day so if you want to go, don't plan on doing anything else. I would not --repeat not--attempt to take this tour on your own. If you do, the ship would have no idea where you are and if something did happen that prevented you from getting back in time, there's a real possibility you could get left behind.
On the other hand, if your ship does offer this excursion and you want to see a beautiful beach / ruins area, then I would go for it.
:wave: wave