Abstract: Flush valves for alternately supplying freshwater or seawater coolant to marine engines. Such valves, used in combination with a marine engine cooling system, facilitate flushing of contaminants. In one embodiment, the valve includes a first portion having inlet conduits, a second portion having outlet conduits, and means for rotatably securing the first and second portions together with minimal gap therebetween. Inlet and outlet portions are alternately positioned in a first, single fluid flow relationship (seawater supply) wherein only a first fluid is directed through outlet conduits, or in a second, multiple fluid flow relationship (flushing operation) wherein both a first and a second fluid are sent to outlet conduits. Also, sealing means is provided between the first and second portions to prevent liquid from escaping through the gap therebetween.