It's really hard to say until that week. We were there the week of December 13th last year and the weather was in the 70's to 80's. We wore no more than a windbreaker at night. I only wore a sweatshirt once.
I was back down to Florida 2 days after Christmas and couldn't get warm at all. We were in Tampa then and that was when the frost covered the strawberry fields in nearby Plant City. We wore heavy coats and gloves the whole time and spent time in the rental car with the heater on high to get warm (we were at a horse show).
Two years ago, we were at OKW during the first week of December and rode on the boat back to the resort from DTD. We all had windbreakers or sweatshirts on, but it was pretty cool. Not unbearable, though we wished that we had gloves.
So, the best thing to do is watch the weather report for the day that you are doing the cruise and make sure that you have appropriate outerwear to keep warm (including gloves). It's a great opportunity to see the Illuminiations show close up, so I'd make sure to be prepared and enjoy the show.
Have fun!
Terri