We have been about 7 times in December and have had a lot of days where it is warm enough to go in the pool, wear shorts and t-shirts but need a fleece as pp said in the evening and early morning.
However in 2010 it was the coldest we have known it and you needed warm clothes all day, even a scarf and hat some days. Although we still got a few days when we managed the water parks over the time we were there.
This year we will be packing a few more warmer clothes just in case