Cozumel's a great place to go, because there is SO much diving there that, if you forget anything or something breaks, you will be able to replace/rent/repair it. (Although looking for a simple brass and glass spg proved to be a bit of a challenge . . . but we found an spg of sorts, at last.) So you don't have to go overboard on the save-a-dive kit. I always carry DIN or tank o-rings, simply because they so often go bad or lost, and it's easier if I can put my hands on my own right away.
I really like having some kind of windbreaker on a boat. You get out of the water wet and the wind hits you, and you begin to chill. If you're diving with Aldora, they have these long, polarfleece lined parkas that are WONDERFUL. If not, a long Gortex jacket is light and very handy.
Most prescription medications are easily purchased in Mexico, except narcotics. So I don't take anti-nausea or antidiarrheals or antibiotics with me.
If you have time before you go, I'd highly recommend acquiring a DSMB and learning how to deploy it. Cozumel diving is drift diving and sometimes the currents are strong; if you get separated from the group, it's awfully nice to have an SMB to put up to help your ascent and to let the boat know where you are.
But above all, don't stress. One of my dive buddies, on our trip to Coz, had his luggage delayed by a day and half. Didn't stop him diving, though!