I have had 9 vacations to WDW in December. I always used the pool, but in the late evening only. Yes - it is a bit chilly getting out of the pool and walking back to the room.
Even though it was a bit chilly, there was always quite a few people in the pool and the hot tub. If it is *really* chilly, there will be steam rising from the pools, which is a really neat thing to see. The pools are all heated, and the feeling is wonderful to be in a heated pool when the air is a tiny bit nippy.