We were at WDW (at the Beach Club Villas) the second week of December this past year, and some days were in the low 60's and some the mid-80s, though more were in the 60s. All of the evenings were in the upper 50s to low 70's. I saw people (not a great many) swimming every day, and a few on some evenings.
My wife and I needed jackets about 1/2 the time during the day, and most of the time at night.
In the past most of our annual WDW visits were made in August because our kids were in school. Since they're both grown now this was the first my wife and I made alone (at least since 1974) and our first in December. The cool weather was a very welcomed novelty, although I missed our usual visit to Typhoon Lagoon.