I'm not queenofcruzin and was a little hesitant to answer your other post. I don't want to be a downer because diving is so fantastic. I just want you to be sure they are certified with PADI or NOWI (if it's still in existence). The reason I say this, you will be using rental equipment. You want to make sure your regulator works, your tanks are actually full, and that you have a dive master with you that knows what he is doing. If y'all are certified divers, it's not as big of a deal. You know what to do in case of equipment failure. If you are not certified, and your dive master doesn't know what he's doing, it could be big trouble. If you're sport diving and only down 20-30', you could most likely make it to the surface if something quit. But you would have to know not to panic. Open water diving is just that, you're in open water, not a pool. Like I said, I don't mean to dampen your enthusiasm because diving is fabulous. I am certified, but snorkel now since it's been so long since I went diving. I would recertify myself if I were to go down again. I do remember how to surface, take my regulator off and breathe from the tank, and most of what I learned, but would rather be safe than sorry.
Cozumel has some great snorkeling/diving. Xel-ha is one of my favorite places, however, it means a really long trek to get to/from there. We've done some snorkeling from the shore in a couple of places that was really good, too.
Have you tried snorkeling? It's really a lot of fun and not nearly as much to learn. If y'all do decide to dive, please make sure they are certified. I'd even ask to see their credentials.