We have been on a Christmas cruise on the Fantasy -- not actually on Christmas, but about a week and a half before. Huge tree (with lighting ceremonies)....big gingerbread house...characters are decked out from time to time in Christmas outfits. I'm a big Goofy fan, and Santa Goofy makes a meet-and-greet appearance. (Tried to let that be our kids' Santa picture for the year, but my wife wasn't as convinced!) One of the coolest things was at
Castaway Cay, where everything is decorated, including the tram, which is turned into a reindeer (red nose, antlers, etc.). No detail left undone.
This was a couple of years ago, and now they're actually calling these holiday cruises something like Very Merry Christmastime (or something like that), so I'm guessing they've done even more. But in any event, they definitely have everything decorated and in the spirit of Christmas.
Castaway Cay was obviously not as warm as in the spring or summer, but we still got in the water and did some snorkeling. A little chillier than in August, but workable. We had pondered taking "shortie" wetsuits, just in case, but it ended up being too much to pack.