As Missytara said, it's the single busiest week of the year. Parks can reach capacity and close by mid-morning. Expect long lines for rides and food.
FL winter weather is extremely variable. You can have cool temps all week, or warm temps all week. You can have a day in the 80's, and then the next day will be in the 60's. You can be pleasantly warm in jeans and a t-shirt during the day, and miserably cold in a fleece jacket at night. There's just no way to tell in advance.
Swimming will, of course, depend on the temps. The pools are heated, but it can be awful chilly getting out of the water if it's a cool day.