A method and apparatus for replacing existing ballast water in a ballast tank of a ship while the ship is underway in the sea includes a seawater inlet port associated with the ballast tank in the sidewall of the ship's hull and admits water into the ballast tank when the ship is moving by producing a pressure that is greater than the pressure of the ballast water that is to be replaced. The incoming seawater from the inlet port is directed into the ballast tank, and an ejector located on a lower portion of the ballast tank discharges the existing ballast water via an outlet port coupled to the ejector that is located aft of the inlet on the side of the ship's hull.