This isn't something that anyone needs to worry about. Speaking as a long time resident of Minnesota (where the lakes freeze over in the wintertime), it takes several days of very cold weather for lakes to freeze over. (Today, as I type it has been below zero for seeral days and there are still several lakes here in the Twin Cities that have not completely frozen over). In Florida, it's much, much more likely that low water levels will interfer with boat traffic, than unseasonable weather.
FWIW, when the recent drought did interfer with boat traffic, Disney provided alternate transportation (bus) to the MK.
Don't worry!