Hi, for that time of the year a 3 mill will do perfect. I live in Cozumel and diving is my business, some people if they just dive for one day they use licra suits, but the better you feel in the water the better dive expirience you will have and that is what is realy all about......so it does not matter if the guy nex to you dives in trunks wille you dive in a full suit or viceversa, remember everybudy is different.
Have fun on your trip.