Truthfully it depends on cold fronts. Dec'03 it was cold, cloudy and dreary for 2 of the 4 days we were there. But the great thing is the bad weather doesn't usually hang out for too long. Disney pools are heated so the water will be nice and warm. A bad thing during the summer when you want cooler water. Once morning in December ( a cold front had come through overnight it was in the 30's and people were actually in the water) I'm a Floridian and not used to cold weather I was very bundled up. We even paid the $8 for black knit gloves probably could have bought at Wal-Mart for 2-3 bucks.
The averages are low 70's during the day and low 50's at night. Its perfect weather. The 50's for MVMCP for us was perfect made it seem more Christmassy(sp).
We've swam in December, even in 2003 when it was freezing. As the PP said the pools are heated. I just brought a long zip up sweatshirt material coverup(really a bathrobe but I called it my coverup). Swimming was wonderful but it was a little cold when you got out of the water.
Visited Florida in December of 2003 and swam at least 3 times that week. My boys got a kick out of swimming in December as we are from Maine and our pool is pretty solid at this time of the year.