We were in Cozumel in April on Princess, so I don't know about this exact tour.
We rented a jeep & drove the San Gervasio Ruins. I'm pretty sure they don't really compare to any ruins like Tulum, etc. If you do go, do not get a personal guide. They talk way too much and we couldn't really understand him anyway. We could have walked around on our own. They were ok, but not grand.
We do like snorkeling at Cozumel. We've done it twice, once at a place I don't know the name of that our cab driver took us to, & last year with the jeep at Dzul Ha. From what I've read on this board, Chakanaab is also very good. The first time we went, which is the one where I don't know the name, but was somewhere a little past Chakanaab, the snorkeling was great. Last April at Dzul Ha it was good, but lots of little jellyfish colonies to watch out for. I think next time we go, we'll do a snorkel tour out of Palankar reef which is supposed to be great, probably on our own, somehow booked over the internet.
Last April, we did really enjoy our jeep rental. We drove all around the island, went first to the ruins, then drove around the far side of the island & admired the beaches, but didn't really stop becuase we wanted to be sure and snorkel & leave time for our daughters to shop downtown. They are real bargainers & love it!
If this is through Carnival excursions I have been on this specific tour several years ago through Sensation. I am from the horse capital of the world so I dearly love these creatures. I found that their horses were ok and they aren't overly strict on how you ride them not that they will let you over do them. However the "ruins" are quite a bit of a joke. If you go, go just to ride the horses just for the pleasure of it. You will learn no wonderful history tidbits here. The guides are nice and entertaining if nothing else as they are quite the cut ups. Would I do that or snorkel if I ever go back to Cozumel.......I would choose snorkeling.