Ixtapa?

[DanL]

Upper 70's to low 80's is the common range for Cozumel. If you are going to be doing 2 or more dives a day over 3 to 5 days, you will probably want at least 3 mil. In Coz last month, water temps were 77 -79 and I was comfortable in a 3 mil with a 5 mil hood. (Don't undereatimate the hood - prevents a lot of heat loss) Others wore 3 mil, some with added vest/hood, and the divemasters were actually wearing 5 mil suits! If its sunny, you really can warm up between dives. If its cloudy and your interval is on the boat in a breeze, you can get pretty chilly.
Repeated deep dives over many days, exposure to the breeze after dives, etc. all result in chilling you down.
Try a 3 mil and have a hat and light fleece jacket between dives if its cool and you'll be plenty warm.