Mercury - Complete Water Pump Kit 9.9-20hp EFI Series Marine-Grade Components 8M0135821. Overview. There's a certain confidence that comes from knowing your outboard's cooling system is truly up to the job. When you're running through a narrow channel on a rising tide or easing into a long stretch of open water, you want to trust that your engine is staying cool, steady, and protected. The Mercury Marine part 8M0135821 Complete Water Pump Kit gives you that peace of mind. It's the kind of reliab