We have cruised twice in December - but on the Magic but we did go to Castaway both times (01 & 02) it was warm and sunny and i do not remember anything specific about the water being cold either. Both cruises were also smooth - our roughest (not counting Alaska last year) was this past September on the Magic but that was courtesy of Jeanne.
I also understand your question/concern as I am sailing on the Wonder in February but - we will have a grand time, in or out of the water.