My boat has raw-water cooled Chevy 350. I installed small ball-valves at the front and back of the exhaust manifolds and on two low spots of the water jackets. I open them periodically and Rev the engine to blow out any debris such as sand. I also use a piece of stiff wire to push open any clogs at the ball-valves. I especially do this in cold weather to prevent freeze damage. I use a wire from one of those little surveying flags.
