I've been many times in December. Some times, it's nice and kind of warm. I'm a wimp - if it's not above 80, no way would I even put on a swimsuit. Kids, whatever! One time, I sat by the pool at the Boardwalk Villas, watched them keep going down the slide. Steam coming off the pool. I'm in jeans and a sweater. One or two other kids in the pool. Finally I said I'm freaking cold, let's go in! LOL!
Honestly, now it's mostly DD and me going to the parks, and we at MOST do one swim time. Last trip, we didn't swim. Most spring break trips, we bring a suit, but don't even bother.
I'd say bring a suit, doesn't take up much suitcase space, but there have been many trips in December where I was freezing. We've had trips where it was down into the 30's at night. You just don't know.