I guess it really depends on what you are used to. If you are coming from a frigid climate it will feel like paradise but it will be way too cold for most tourists to enjoy any of the water parks. The water temps will be too cold in December.Expect the weather to be variable. It is very possible to be well below 40 degrees one day and above 75 the next day.The average temperature in Orlando in December is just under 73 degrees.
Christmas in Orlando varies GREATLY from year to year -- I ve lived here over 20 years and some years I m bundled up in what looks like ski clothing and other years I m sweating in shorts and a t-shirt. As for the water parks, the water is heated in all Disney parks and pools (to 74 degrees, I believe), so it s really a matter of whether the air temp cooperates. You might watch Accuweather.com for forecasts (they go out 15 days -- the furthest I m aware of for any weather site) and get an idea of what the weather s going to do, this year.
My sister and her family were in Orlando last Christmas. They were able to spend time in their villa pool on Christmas Day. The evenings were cool but the days were lovely and warm. I don t think they went to a Water Park but even my 88 year old mum (who was there too) said it was nice and warm.Have fun!
here is a website dedicated specifically for christmas in orlando... have a great time!http://www.christmasinorlando.com/