Totally depends on your luck with the weather ! Our very first trip was in Dec., right before Christmas, and we could have swum every day -- it was nice and warm. We were too busy doing the parks for the first time. Our last trip was Jan. 2 - 9 ,2003 and I'm pretty sure the ice at Blizzard Beach was REAL ! LOL We wore layers and polarfleece jackets and gloves every day --I couldn't imagine swimming. My DH and DD went to the pools at the Poly several times; even with the volcano pool and the quiet pool being steps away from our ground floor room, they turned into icicles.
As previously mentioned, check the schedule for rehabs. Have fun !
Leslie