We go to Cozumel a few times a year. I can't think of a single trip that there has not been a single diver on the boat at one time or another. Off the top of my head...an Engineer from Mexico, a Nurse from Maine, a family lawyer from Dallas, etc. The Good thing about Cozumel is the warm, friendly people and friendly atmosphere are contagious. Any dive ops with multiple boats will be able to make sure that you are on a boat appropriate to your skill level. Spend an hour or so on the Cozumel forum on this site and you will have a lot of ideas on where to stay, where to eat, what the favorite reefs are, who the favorite dive ops are, and about anything else you would like to know.
If you go once, I promise you will be back. The feel and the people are nothing like mainland Mexico. The other great thing is you can spend as little, or as much as you want. A weeks diving, with air fare, for me and my wife usually runs right around $2000.
Best thing about diving in warm water is you can actually see all the other fish in the sea!! They are out there....you just have to catch the right one.
Sorry for the "all people in the Pacific NW love coffee" stereotype, but the best coffee I've ever had is at El Coffee Cozumel!! If you don't love it, I'll buy your cup!!!! Maybe it's the location....,
Good luck and safe travels.
Jay