We have been to Orlando for four vacations........twice in January and once in December (the other was during August). We swam on every trip.
Heck, we even went to Aquatica on Christmas Eve day this year.
Funnily enough, alot of waterpark and pool staff are apt to remark "so you're notherners, huh?". Apparently, like the PP said, those born and raised in the south often find the "winter" months too cold to swim.