Whether you need a wet suit in Cozumel is entirely up to your own reaction to the water temperatures. Everyone is different. Some people see no need for a wet suit, but I know two instructors who use a 7mm suit there. One of the dive masters I used this summer used a 5 mm suit.
As for me, I used to be pleased as punch with a 0.5 mm suit there until it got too baggy. I now use a 3mm suit. I always wear full suits in the ocean, because I never know when some little jellyfish or something similar is going to try to make my life miserable.
Wearing a wet suit out of the water in the tropics is a really good way to get heat exhaustion. Bring it with you and then pay attention as the boat heads toward the site. At some point, if there are people who are knowledgeable about the area on baord, you will see them start to suit up. That's a good clue for you. Otherwise, the DM will usually tell you when to start getting ready.
The dive operators in Cozumel are very helpful, almost to a fault. Let them know your experience level, and they will make sure you are OK. In addition, let the operator know your experience level when you sign up for the trip. If they have a number of boats, they will try to separate divers by ability. Many of the dive sites there are of the kind that a boat full of beginners and a boat load of experts can splash in at the same location and have totally different dives, with the DMs using their judgments to lead each group as is appropriate to their abilities.
