Here is a report from Cozumel.
General Weather Info:
The temperatures in Cozumel are at their coolist from October to April.
During the day it is in the 80's. At night during a Norte, it can get down to the 50's.
Summertime can be pretty hot. Although the temperatures are in the 90's,
it is the humidity that gets you.
It tends to rain more in June than most months, but the really dry times are in winter.
Nortes come during the period of October to March. They can bring rain and winds,
but they do not last long.
The water temperature is warmest from April to September. The waters can get as
warm as the mid 80's. During the winter, it will cool down to the high 70's.