We went in January in 06, 07, 08 and 09.
You need to be prepared for warm and cool, we even bring thin gloves for Spectromagic, etc. Most trips we can typically make it in the pool a few days and we get to wear shorts about half the trip, but 09 was so cool DD is wearing the same 2 fleece tops in most of our photopass pics! Every year we had very little rain, 09 I think only one afternoon.
This year I followed the 10-day forcast and packed ahead, a day or two before everything changed with a cold-front and had to re-pack. Still brought some shorts and dresses just incase, only the first weekend was warm enough to wear them. I don't mind the Jan temps... it beats snow and ice!
I wouldn't buy waterpark tickets until you get there, you can upgrade tickets if the weather is nice.