Hi,
We have been in January the week following new year (for two weeks) and at the end of January this year. It looks like we will be out there the same time as you next year (due to my birthday!!).
The weather in January is unpredictable. We have had temperatures in the high 80s and we have had near freezing temperatures. It has ALWAYS been bright and sunny though, whatever the weather.
Generally you need to plan for cool to cold evenings and early mornings and warm to hot afternoons. Layers are the way to go!
I generally find that I am fine in jeans and t shirts with a sweatshirt just in case. We have always managed to find a day for a water park and last year did Discovery Cove.
I hope this helps