I don't think I did a very good job trying to answer your question. Do I get another chance?
Squarepants brought up some good points too.
When you email, or call an 800 number to make your reservations, do it for the days you want to dive.
That way the operator knows how many people they have for the boat that day.
When you arrive in Cozumel go to the dive shop.
Bring your C card, and if you're getting Nitrox bring that card too. Bring your money.
Tell them which days you made reservations for. Hopefully you received a conformation number or someone's name.
Then they will ask to see your cards, have you sign the waivers and pay for your dives.
If you don't make dive arrangements before you go, you have the chance of the boats being full when you want to dive.
Fall is off season in Mexico. But, that's not to say people don't go in off season. People do go in off season. It's off season because of the weather, and because kids are in school, and parents do not travel as much then. But, Divers go to Cozumel all the time.
Make your plans ahead to be sure you're diving!