KevinP wrote: As someone who lives in Florida...
This is exactly what you have to take into account.
I've been to Miami and the Bahamas in January a few years ago. Every day it was above 75 Fahrenheit (24 Celsius) or more.
For someone that lives in Florida anything in the 60's might feel chilly, but for people that are not used that it ever gets over 90 easily, that is really warm, over all during the winter...
Plus: aren't we going straight south? So Cozumel should be even warmer. I found this here: http://www.cancuntravel.com/weather.asp?d=cozumel
Cozumel: Average daily High in January: 82 Fahrenheit (28 Celsius) ,average daily Low in January: 68 Fahrenheit (20 Celsius)
Sure, weather is unpredictable, but think positive and don't forget that lot's of people will be coming from places that are covered under tons of snow and shit like that.