If your port was some place other than Cozumel, I would say book from Carnival, but Coz is so easy to get around, and it is almost impossible to get lost.
Do you have any idea what you want to do in Coz? I have a few names of tour groups I could share if you have an idea what you want to do.
Have you checked out the excursions that Carnival has to offer? Lots of times we can tell you someone else to see about the same excursion, for less money.