Mornings and nights can be on the cold side. The contrast from the afternoon gets to you which makes it feel a lot colder. We always layer our clothes in December. We will have hoodies, sweaters and even jackets during the cold time.
The pools are outside and they are heated, but I just feel a chill thinking about getting in and out of a pool during December. It's not for us. If you are going to try this I would certainly go in the afternoon when temps are the highest. If you run into better weather then enjoy the pool, but just be aware it can be cold.
We won't go on water rides, either in December.