Mercury - Water Pressure Sensor Kit Engine Monitoring Series Composite & Brass Assembly 8M0157742. Overview. When you spend a lot of time on the water, you start to notice the little things - the way your engine sounds when it's running right, the way the hull feels when it's trimmed just so, the sense of calm that comes when everything is working the way it should. A water pressure sensor is one of those small, behind-the-scenes parts that keeps that confidence alive. Mercury Marine part 8M0157