We went on the Magic - Eastern last December. The weather in St. Maarten and St. Thomas was great. Temps in the 80's, sunny and warm enough for swimming and snorkeling. I believe the same is true of the ports along the Western route.
The only place you might have cooler weather is at CC or in the Bahamas if you're on the Wonder. Cold fronts can occasionally push that far south and out to the Bahamas. When we went to CC, it was a little cool. Temps in the 70's and very windy. We went in the water a little but it was really cold. It was still sunny and a nice day so we enjoyed the island out of the water. DH played volleyball on the beach and I took the kids on a bike ride. It was also very cold when we disembarked the following day in Port Canaveral. (It was actually warmer when got home to Texas than it had been in Florida.)
I think that this is rare though, and most people who go to CC in December have warmer temps. We had a wonderful cruise and have booked another next year at the same time!