Just a thought on this subject. On my boat, stainless water tank. The overflow for the tank comes out at the side of the bow. (low tug deck) when in some locks with leaking gates, paddles etc.The canal water often comes in close proximity to the overflow. Don't know if this canal water could enter the water tank this way? Should they be a one way flow valve /flap on this overflow pipe. π€. Must have a look next time I'm at the boat. Anyway to check it. Possibly putting a hose pipe into the overflow outlet and see if the water tank fills up. Never had a problem drinking the water be for.