We go to Disney every January, and while there are generally a few days warm enough for swimming, we've never had a day that was hot enough for us to want to spend an entire day at a water park. I agree with those who have said the weather is very unpredictable. We've had temps in the 40's, in the 80's and everything in between! Basically, we have to pack for every season when we go.
DH and I are planning a short trip in September just so we can enjoy the waterparks and Stormalong Bay, which we don't normally get to do during our winter trip. I'm looking forward to the change, but will probably be anxious to go back to our annual January visits when we're not melting from the heat and humidity!