Some may be closed for rehab but other then that they are open. We were at the Poly in Dec and we had lovely weather but the mornings could be chilly. Around 50 and there were people in the pool!!!
The pools are heated so they stay open all year unless, as mentioned, they are down for rehab/maintenance.
If you are using one of the quiet pools at the resort there is usually a laundry room close by. I take a towel from the room and just before I get my DD out of the pool I throw it in the dryer to make it nice and warm then wrap her up in it for the trip back to the room. This is great on those chilly Florida nights.