What would non-potable water be used for on an airliner?
Aircraft travel in countries were pathogen bacteria or viruses are carried by insects which like to lay their eggs in stagnant water. Aircraft water systems must be designed carefully to prevent spreading diseases.

On a typical airliner like the Boeing 737NG, you won't find any non-potable water source that would contain chemically untreated water. Equipment is fed from the potable water tank. Used water is either dumped overboard or stored. From the aircraft familiarization manual, the water system:

  

and the toilet waste system:



Condensation water is a by-product of many systems cooling air (including by expanding it). This water is collected and drained overboard. Jet engines (including APU) are the main generators of water, as fuel combustion generates essentially water (in mass) and CO2.

It's worth noting that at cruise altitude, moisture in atmosphere can be insufficient for humans well-being. The water extractors present in air-conditioning packs then reincorporates a part of extracted water into conditioned air.