Salt water is harder than fresh water on just about any media. Special extruded aluminum alloys are used in some yacht hulls but they are the exception. Cast aluminum like exhaust manifolds and shaft logs will corrode at a much faster rate than in fresh water exposures. What you are doing is the absolute best thing to do if you are boating in salt water. Next is the importance of removing your boat from the water overnight, electrolysis in salt water is an even bigger threat as our boats are not grounded for that application.
