Hi Sofia,
It is possible to enjoy the water parks and the pools at Walt Disney World all year round as they are heated. However, you do have to get out of the water at some point! That's especially true at a water park where you go from one attraction to another attraction.
The average high in January at Walt Disney World is about 70 degrees. That's the average which means there are days much colder than that! I suggest checking the Weather Channel at www.weather.com for the days you are visiting.
The good news is the crowds will be much thinner in January than they are in July! Also, both water parks are absolutely fabulous! Their theming is amazing!
Just keep in mind that
Typhoon Lagoon is closed for refurbishment until January 9, 2010.
Blizzard Beach is scheduled to be closed for refurbishment from January 10, 2010 through March 20, 2010.
Have a magical vacation!