Abstract: A fresh water tank or sump is positioned with respect to a salt water or seawater supply in a manner such that the level of fresh water in the sump is substantially higher than the highest expected level of the salt water. A salt water supply pipe is connected into the fresh water tank at a point well below the normal level of fresh water in the tank but at a point above the suction of at least one pump adapted to pump water for fighting fire from the tank to a fire fighting water supply system. The seawater supply pipe is equipped with a flap or equivalent valve which is maintained closed by the hydrostatic head of the fresh water which is higher than that of the salt water at said flap valve.