Are you going to Mexico or Belize via the Gulf of Mexico? The weather will still be warm in December.
Last Dec, my family and I went to Mexico via the Gulf of Mexico and it was warm, 78-85 degrees, humidty was up and you could swim if you desired. I am from Florida and it does not really get cold here until January.
It will be warm in the ports probably but Galveston could be chilly. It could be 30 degrees and it could be 70 degrees. It just depends - I've worn shorts on Christmas but then again I've had a heating blanket on the bed a couple of years - we are in Houston. Debbie