Hi,
We visited WDW for a quick 3 day stay while passing through to Miami 3 years ago. We were there late Dec. for New Years eve. Our weather was beautiful and warm, shorts during the day and a sweatshirt at night. We ddidn't visit a water park due to the short stay but the weather would have been just great for a water park visit. We did swim in the resort pool everyday. You just never know the weather that time of year. The year prior we were there slightly earlier in December and nearly froze, had to buy fleece jackets and mittens. Watch the weather and be ready for anything. We are going down over Thanksgiving this year for 10 days, I am hoping for warm weather as well with hopes to visit a water park. We'll just have to keep our fingers crossed. Have a great trip.