I have been to WDW during early December on a few separate trips. I can tell you that the weather is completely unpredictable!
One year we were bundled in sweatshirts and jackets. It can also differ from day to day. For example in 2007, December 6th had a high of 67 and a low of 51. That is not swimming weather, even for us travelers who venture to Orlando from the North.
However I was there on December 11 when the high was 78 and low was 62. The humidity was also 100 percent. It made for hot and humid afternoons and very pleasant evenings. Definitely nice for swimming.
In 2006, the week of your planned vacation had some highs in the 70s but some highs in the 50s!
It's variable and unpredictable. The pools are heated to 84 degrees. Even on what I consider cool days, my son has often gone swimming. It's quite amusing to see the lifeguards bundled in blankets while the kids from snowy northern locations enjoy the pool.
Watch the 15-day forecast via
www.accuweather.com to help with your packing. And pack layers!