Hi,
You should be aware that one of the water parks is always closed in December for refurb.
Also, December weather is complettely unpredictablke. You could have warm days (80s) with cool mornings. Or you could have mornings near freezing with afternoons in the 40s or 50s. Or you could have little of both, or something in between.
To sum it up: when you go to WDW in the winter months, you should not count on being able to swim, but you shouldn't rule it out either.