It will be pretty crowded until the Sunday after Thanksgiving, then it tapers off fast. We have been there the week after Thanksgiving four times. It use to be a ghost town in the early '90's. Attendance has picked up, but doesn't compare to holidays and summer.
Weather is totally unpredictable. We have never experience HOT weather, but have and a full range of everything else. I think the average daytime temp for December is 73. We would sometimes get a couple of pretty chilly days and once it was downright cold (50's day 30' night). Just remember that's not typical, but it is possible. We have always been able to get a day warm enough for a water park. Lowest month for rain. As far as packing, check accuweather.com before you pack. They have a 15 day forecast. That way you can pack appropriately and not have to bring everying you own! We have never brought winter clothing (I refuse!!) and a couple of times we bought sweatshirts in the evenings when we got caught unprepared in the parks for chilly weather, but we're from Michigan so even 70 is wonderful in December!!