I'll be on the same cruise! A few years ago we did this cruise during the first week in December and I definitely found it chilly - it was too cool to be comfortable in swimwear at Serenity Bay, for example. I just used towels as blankets all day because I refused to go back on the boat and lose time on
Castaway Cay! It was still a fantastic day.
I've done a little research on average temperatures in the Bahamas this time and it looks like mid to upper 70s are fairly typical highs, so temps were lower than usual on my last December trip. Since I'll be traveling from Chicago to PC, I'll have a pair of jeans and sweatshirt with me anyway. IMO, I think it would be smart to have one pair of pants and at least a long sleeved shirt with you, just in case it's cool in the evenings. Hopefully you won't need them.