The pools are heated to 80+. They are plenty warm when it's less than 80 degrees outside.
(I haven't been to Volcano Bay, nor any waterpark, other than in the summer)
We swam during our first trip December 2010, when it was 50 degrees. We did not swim when it got colder. And that 50 degree evening was miserable for the parts of our bodies that weren't submerged. I don't really recommend it unless you're Scandanavian and are used to cold plunges.
But in normal late Fall temperatures? Swimming is fine.