Raw water engines aren't dedesigned for saltwater, and I don't think anodes would be on any of them. Even with anodes, the manifolds don't get the protection needed because there's no water in them when the engine is not in use (which is the vast majority of the time), and anodes must be submerged in order to work. I believe manifolds typically are replaced in say, 4-5 year cycles. There are SS manifolds on the market which offer a long life. There's no reason why you couldn't install a pencil anode on the manifold, but it wouldn't do much because of the above senerio.