The diving in Cozumel is some of the best in the world. The people are kind, cheerful, unfailingly polite, gracious, and will go out of their way to help you. Tip generously, be gracious and polite in return, pepper your speech with "Muchas gracias," and "por favor," and you will have the most excellent vacation you've ever had in your entire life. In general, Cozumel is a very friendly, laid-back place, and if you behave with a little class and mind your manners, the people are some of the best you will ever meet.
This has been my experience, each of the three times I've been there, and I can't wait to go back.
I was there for Hurricane Wilma in October of '05, at the Casa del Mar. Many of the staff and their families were staying in the hotel, and waiting to eat until the fifteen of us touristas had gone through and gotten our food. (The served us three delicious meals a day, even on the most basic of supplies, and they were going out in the height of the hurricane to bring more supplies from the trashed restaurant.)
We said, "No... That's not going to fly. The staff and families need to get in line with us. We're all in this together."
Those people are like family.
I love, love, love Cozumel and the people of Cozumel.