we can have 2 carry-on bags each, 22lbs TOTAL weight, 22x16x10. We will use standard size duffle bags, which should come in just under the size restrictions
We are just open water divers, using air, and staying above -60 feet elevation except for the rare occasion to chase a mermaid deeper
This is from my perspective; a recreational diver who likes to dive when he travels - not one who necessarily plans a trip just to dive. In Cozumel, you can pretty much plan your trip anyway you want, especially if weight/size is a factor.
I travel with a large, hard case, checked bag (maybe 31"x20"x13.5") and a hard case carryon, the size of which you described. For Coz (really anyplace), I can get all of my gear and clothes split between both. Usually around 45 lbs in checked bag alone, so
I don't see how I or anyone could take all of their gear in 2 smaller cases - and I have a travel BC and don't carry near what I guess most divers, who have replied, take with them.
The only thing I really feel I must have is my mask, because it's prescription. Having my own gear has saved me quite a bit of money vs renting, so if I had to rent, it wouldn't be a big deal. I could survive diving a week under the conditions you described with rental gear. I was surprised how many divers that I was on boats/ in contact with recently in Coz, actually rented gear. A BC, reg, and computer would be the next most important things I took. After that, I could rent - don't have a hangup on other people having used it.
Wish I could help you out on Scuba Club and their rental gear - I think they have lockers (without locks) Some of the other ops have tubs and/or hanging racks where you can leave your gear ,including rental gear, and they store it for you. You might call Scuba Club and ask them.
I'm headed to Cabo and since I only have a 3mm and shorty, I'm renting a 5mm while I'm there - actually it will be free as long as I dive 3 days with the op!