I asked the "pool guy" about this last June, as there was a "Bio Hazard" left behind by an elderly guest in the adult pool (it happens). He said they drain them to be cleaned for such occasions, and they get a bleach washdown. The pool was re-filled within the hour.
He also said they re-circulate in fresh water throughout the operating day, to keep the suntan oil, hair, etc. in check. As you know, the Magic and Wonder make their own fresh water from seawater.