I just picked up two sets of exhaust manifolds and risers for my 454 MerCruiser engines. I have a question concerning the gaskets that go between the exhaust manifold and riser. There are two kinds of gaskets. I am not sure what gasket to use for my exhaust. One set has some of the water passages blocked off while the other set has all the openings for the water flow. I mostly use my boat in salt water. Which set of gaskets am I supposed to use? The ones with all the water passages open or the set with some of the water passages blocked off? If I am not clear I could attach some photos of each type of gasket.