Here is a link to a trip report I wrote last year when I went to Cozumel at the end of January into the beginning of February. My wife and I stayed at Fiesta Americana and we used (and highly recommend) Christi's dive operation, Blue XT Sea:
http://bluextseadiving.com/
Trip Report:
http://www.scubaboard.com/showthread.php?t=86797
I wore a 3 mil wetsuit for all of my dives. My wife wore a 3 mil shorty with a beanie/hood for her dives. I think you should bring a wetsuit, although like previously stated, personal comfort levels vary.
Some dive operators will provide you with a windbreaker and dry fluffy towels for your surface intervals, so you might not need to bring your own. Check with your operator. If they don't provide, you'll be happy if you have that with you for your surface intervals, so you should bring.
As someone else said above, biodegradable sunblock is good to bring. Knives and gloves are prohibited.
For the evenings, shorts and a short sleeve shirt will do just fine, with sandals. In case any particular evening is a little chilly, a pullover anything will suit you just fine if you're cold.
Bring all of your own gear as opposed to renting. Your operator provides tanks and weights.
Eat breakfast at your resort before diving. Lunch could be either at the resort or in-town after your diving. Every night, you should head into town to eat. Take the taxi.
Use your credit card at the restaurants and any stores you buy from. I used cash for the taxi, dive operator and tips. You can always get more cash from a local atm.
Hopefully, your dive operator will store your gear for you, like mine did. But bring a smaller bag with you on the boat for any of your things that you'll take back and forth with you every day.
I'm sure there's lots more people can tell you to help out. Enjoy your trip.
Michael