A fellow Kentuckian, eh?
Is the Iberostar an all-inclusive resort? At least whatever package you guys booked?
In the unlikely event you got separated from the main group, it'd be nice of have a safety sausage/surface marker buoy. Fits in a BCD pocket rolled up. Ideally it's nice to have on a reel so you can inflate at depth and send it up to reduce risk of getting hit by a boat surfacing well away from the dive boat, but at least take one. Cozumel diving is drift diving, and any large water body merits an SMB as at least a good idea. Little diver heads don't stick up far, they're hard to see from boats, and I hear sound doesn't travel well so don't assume those diver whistles will save you.
Pack a mask that fits you in carry-on luggage. Most gear can be rented if the airline loses your checked bags, but mask fit in personal, hard to know before you dive, and a leaky mask is a hassle. This wisdom got drummed into me the hard way on a North Carolina trip... If your dive computer has a proprietary battery, maybe pack a spare.
Somewhere on the forum is a thread on forbidden items on Cozumel trips. Somebody got busted and hassled for Sedafed, if memory serves. I rely on 12 or 24 hour Sedafed to keep my ears clearing well, so I'd want to, um, 'find a way to get my needs met.' Do they sell pseudo ephedrine in Cozumel itself? Some drops to prevent swimmer's ear are worthwhile when diving a lot; start using them before you're in pain, or they'll burn.
Leaving Grand Cayman I gave up a scuba dive tool as I learned they didn't want to let me on the plane with a carry-on with a 'tool.' Also don't pack a dive knife in carry on.
If you like taking notes, one of those little 'wet notes' type pads can be nice on a dive boat, where things tend to get wet. Ask if the boat has a big community dry box/trunk; if not, you may want a little hard dry box to put your phone & such in while you're diving. You might ask if it's a covered boat (shade), and take sunscreen regardless.
When trying to figure out what there is to do at a foreign destination, a resource I use is TripAdvisor.com. It's fine to ask here, too, but people like to do different things. If your dream excursion is strolling through a botanical garden, hearing about the best bars may not help. I don't ride scooters; I've seen people warn against newbies trying scooters in Cozumel. So, if you're picking out excursions, this might not be one to do.
Most of what I told you is about scuba tourism is general, not specific to Cozumel. Happy diving.
Richard.