Disney pools are heated.
December can be hit or miss. Sometimes its warm enough that everyone from the mid-South North swims (people from Texas watch bemused in their jeans and jackets), other times its just people from Michigan and Minnesota in the pools (people from Kentucky watch bemused in their jeans and jackets).
Even when the Disney gardeners are working overtime due to frost, you always see some people in the pool - when its below zero back home, anything short of having to chop through the ice to swim is ok.