Can not answer for the christmas week but we were on board for new year last year and for us it was amazing. Tha atrium was decorated and yes it snowed there too! we enjoyed it soo much we are again coming back for this new year.
We were on the Sea Princess last Christmas/New Year. There was a formal night on Christmas Eve, but Christmas Day was very low key, the attire of the day was casual, possibly because we were in Tortola. I know a lot of people thought this was disappointing and wondered if it was an American thing. However, New Years Eve was wonderful with a deck party and the Captain discovered that a large firework display was taking place in Curacao, so anchored the ship off there at midnight. You will have a great time as Christmas in the Caribbean is special and you will still get crackers, mince pies, Christmas carols and even snow in the Atrium! Also, lots of people take decorations for the cabin, if you like them.