Which are the requirements for the water supply on board?
The IATA standards as per the IATA Drinking Water Quality Pool are based on 


  ISO 17775, WHO requirements or EU/98/83/EC or those set by the civil aviation authorities of the participating airlines


Source

Airlines and operators will periodically test their water in aircraft, ground service vehicles, and filling stations for bacteria contamination (E-coli, coliform, etc).

Some aircraft may not actually have potable water tanks or may have removed them. Horizon Air operates their Q400 Dash 8 aircraft without potable water.