We have been twice in December. One time the weather was pretty cold. When we got to CC, it was actually warmer in the water than on the beach. People were all bundled up. It was not the best.
This passed December, the weather was beautiful! The water was a bit cold at CC, but it wasn't too bad in Grand Cayman. It's not unbearable really. The beach and water were full. Lots of fun. Just cross your fingers that the weather is nice.
Have fun! The decorations on the ship are beautiful!
My advice, don't order your Christmas (holiday, whatever) cards until the night before you leave. Look over your pictures, including the ones you get on CC. Then get them made when you get back on the ship. Even though everyone looks awfully sharp on formal night, we found that the casual one with Goofy in front of the ship was the best.
Kitty