Yes, I can now say, yes I have been to a water park in winter.
After a great many WDW visits, the only OTHER time we went to a water park was in August!
Here's the funny part: our AUGUST water park weather was COLDER than our December visit!
Where weather is concerned Orlando can be surprisingly unpredictable and uncertain. I never though we'd be chilly in FL in AUGUST. Likewise, this past December was far warmer than most of our other winter visits (a very many).
The temps are not likely to be very warm when you go, but you never know. I suggest trying to be as flexible as possible to find a warm day. Try to allow at least two days in your schedule that could be used for the water park, maybe one early in the trip and one late.
The water is heated to a decent temp, and I am a cold-water wimp. There are no hot tubs though. I am not sure if any of the dressing rooms have showers. Between slides you could get chilly. The Typhoo Lagoon shark reef water is chilly for the wildlife.
The other thing we kind of discovered is in some ways the less you bring, the better. Towels, yes. Valuables, no. No cell phones. If you have a car, it is good to drive. Water shoes are wise. If you don't have them, flip flops are wise. You don't want to use the restrooms barefoot. A full day of walking barefoot though does a number on our feet! Sore feet +WDW is a bad combo! so going towards the end of your trip might be best, if weather allows.
The layout is kind of large, so you kind of drop your stuff, then wander around. You can't wear flip flops on the slides. But you can wear water shoes.
If you are slightly brave weather wise- like all parts of WDW- you'll probably be rewarded with sort lines! Though the park was far from empty in December, we managed to go on quite a few slides in just a few hours.