Mercury - Quicksilver Water Shutter Exhaust System Component Flexible Check Valve 60930. Overview. Keeping water out of your engine is critical - and that's exactly what the Mercury Marine 60930 Quicksilver Water Shutter, also known as a water flapper, is designed to do. This flexible valve fits inline in the exhaust system of MerCruiser sterndrive installations, closing when the engine is off to stop water from backflowing through the exhaust and into the cylinders. When the engine is running,