The pools at WDW are heated but the temperature can vary from one pool to another, even within the same resort. We stayed at OKW last January and the pool near our building was much warmer than the main pool, perhaps because our local pool was smaller and more compact. But even the main pool at OKW was warm enough for a swim. SSR has several pools so if the one closest to your room feels too cold for you, check out the others in case one of the other pools is a bit warmer.
Coming from cold New England, we love being able to go swimming at WDW in Dec/Jan. Have fun!