Mercury - Water-Separating Fuel Filter Fuel System Protection Marine Coated Steel 866594K01. Overview. Nothing spoils a perfect day on the water like contaminated fuel. Even when you purchase from reputable marinas, water and debris can still make their way into your tank. This is where the Mercury Marine part, number 866594K01, comes into play. This filter works quietly to protect your engine every time you turn the key. It keeps water out of the fuel system, safeguards the injectors from damag