December is an unpredictable month. When we were there last year we had days warm enough for the water and it was cold on other days. Plus if I remember correctly one of the water parks was closed down for refurbishment when we there. It depends on what the weather is like whether the park will open up for the day. If it is going to be 50 outside noone is going to be swimming.
I am sure the park will be open one of the days while you are there.
We had more warmer days than cool days while we where there, but did not do any swimming, my DD had an ear infection on the day we arrived so swimming was out of the question.