I took my first cruise earlier this year. It was Carnival, but out of Tampa. We stopped in Cozumel, but I am glad I didn't try to cram in diving as a shore excursion because the amount of time our ship spent in port was on the short side of what appears to be average. I learned to not just make sure I have enough time to do the shore excursion but enough time to feel comfortable and relaxed overall, as I am the type who despises being rushed. Carnival's shore excursion desk may be glad to sign you up for diving, but if you don't like the feeling of being rushed I suggest you do your own research and figure out how much time YOU feel you will need. My wife and I opted to only go snorkeling instead, and we still felt rushed. It wasn't so much a matter of risking being "left behind"--that's hugely unlikely--but rather a matter of not feeling we were being herded here and there like cattle. We were glad we did not sign up for a dive, even though diving was presented as a shore excursion option. We had been to Cozumel before for diving, and we will be back, but not on a cruise ship.
From what others have said (
see this thread I started), different ships might spend different amounts of time in the same port, so you have to check for yourself and determine if you feel that is enough time to enjoy what you plan to do. Some ships apparently spend a generous amount of time in Cozumel.
If you decide to dive, and Cozumel is going to be your only dive, I suggest you rent all your gear. I can't imagine lugging dive gear in addition to all the crap I had to bring on that cruise. A cruise is not like a dive liveaboard, where all you need to bring in addition to your dive gear are swim suits, a pair of shorts and some t-shirts. All in all, I found cruising really wasn't my cup of tea.
