The weather depends on your itinerary. If you're on the 3-4 day cruises, the weather can be cool in Dec-early February. Our Jan 2003 Wonder trip went like this: Port Canaveral high in the low 70s on departure day, high in the low 60s on arrival day (followed by the next day which had highs in the low 40s, with lows in the high 20s

) Nassau had highs in the mid 70s, CC had highs in the low 70s. It was too chilly for me to swim, but the kids were right in there.
7 day cruises go further south, so the temperature is typically warmer. My Dec 04 experience was low 70s in Port Canaveral on arrival and departure, Key West was also low to mid 70s, but a pretty good breeze. The eastern ports were in the low 80s and beautiful sunshine! We had a bad CC day, temps never got above the high 60s, and it was gray and windy.