I was on a 5 night on the Wonder in early December that went to Cozumel and then CC. We had beautiful weather (rough seas, though) but the Fantasy had been unable to dock at CC the day before we arrived at CC because of high winds and rain...so don't count on consistent good weather...
We vacation yearly in Cozumel. January is our month but we are going late Dec this year. It still can be pretty hot with a passing shower. If a Norte comes through you can expect wind and cooler temps.
We found CC on the cooler side in December. Warm but not hot, and when the sun was behind the clouds it was downright cool.
Same thing for January. Water was too cold for swimming.
Cozumel was great. Pretty hot.
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