i was there last few days of december and until the 4th of january this year.
First few days we were in town it was nice and hot well into the 80s. Then it got colder after a daylong rain and only made it into the 60s. And it stayed cooler for a while.
Disney only keeps one water park open then and if the weather is like that, they don;t bother to open the water park sometimes. When temps are in the low 60s as the high and it is still in the 40s in the morning, the waterpark is not the place many want to be. Unless you're from the Artic. I gotta think it takes the water a few days to warm up after a cool spell like that too. I think the "average" high that time of year is about 70 degrees.