Abstract: This invention discloses a seal arrangement between an outdrive unit and a hull of a vessel, comprising an outdrive unit connected to an engine and mounted on a bed of the hull, the outdrive unit projecting downward through an opening in the hull; a diaphragm seal fixed to the hull and the outdrive unit for sealing a space between the hull and the outdrive unit; an annular seal flange projecting above the diaphragm seal from the bed toward the outdrive unit; and a ring seal arranged between the outdrive unit and the seal flange for sealing a space therebetween; characterized in that; both ends of the ring seal are fastened to the outdrive unit and the seal flange, respectively. A water sensor connected to a warning device is arranged in a space between the diaphragm seal and the ring seal.