Actually in general only one of the water parks is open in December. One usually closes for refurb in late October and reopens in early January and then the other will close between January and March. Plus both are subject to close if the temps are low.When we were there in December, it was easily warm enough to swim but I understand the temperatures can vary. As someone said previously, the pools are heated so temperatures in the 70s still result in a nice swimming experience. It probably tells you something that generally the water parks are open in December but may be subject to refurbishing in January-February.