Abstract: An apparatus for filtering, retaining and disposal of waste water accumulated on a boat. The apparatus includes a tank and a device for controlling the flow of waste water from the tank. The device includes a depth measuring instrument responsive to water depth below the boat. The depth measuring instrument provides a depth signal representing the water depth below the boat in relation to the a pre-determined water depth. The depth signal is received by a signal generating device operable to generate a discharge signal in response to the depth signal. The discharge signal is received by a discharge device which discharges waste water from the tank in response to the discharge signal.