We dive Cozumel 2 to 3 times a year. The operator will make a big difference in what goes on. We dive with Aldora for the steel 120 tanks and insane bottom times. I have dove with Dive With Martin and Blue Magic as well and have nothing but great things to say about them. Christi with Blue XTSEA is a huge supporter of this site as well and deserves a chance for your business.
If you go to the Cozumel Forum, I had a write up not too long ago about the differences in operations. As far as what to expect, awesome diving and great service! An operator with several boats will group yall according to ability and choose a site that meets your ability. Make sure that they are aware of any limitations, if any. Depending on how many of you are diving together, it may be just as cheap to charter your own 6 pack boat. Stay away from any operators that have more than 6 to 8 divers on a boat.
I usually take my bcd off in the water and hand it up. My back, shoulder, and knee aren't what they used to be. That's never been an issue. Entry is backroll, but they can help with a giant stride if needed.
The diving will likely be drift diving in slow current so there is little physical exertion at all. Lots of beautiful reef to explore, lots of sea life, and great diving at any depth you choose. The people are warm, genuine, and friendly and great food at what ever you feel like spending. We have walked all over town at night and never felt unsafe. We can go other places....we choose to return to Cozumel time and time again.
Again, check out the cozumel forum on this site...more reading than you can do in a week about dive ops, conditions, lodging, food, shopping, food, car rentals, food, and oh yea...food!
Good luck, safe travels,
Jay