The cooling system - cooling water will drain (equalize) out of the engine over time back through the impeller and on a N30 with the MD7A will end up with ~ 2" of air in the top of the engine cooling system. Salt will precipitate out at this point and plug water passages. Rust will also be accelerated above the waterline. Shutting the inlet valve while away from the  boat to keep the engine full of water will prevent this. Flushing the engine with fresh water after each use will help disolve some of the exixting salt deposits.

I have modified my boat and now flush the engine and close the valve when leaving the boat for the week or more. I hang my engine key on the water  valve handle so as to not forget to open it before use. I ocasionally fill  the engine with vinegar to dissolve salt deposits. The flushing system makes this easy.

Flush out modification - Tee in to the outlet side of the water strainer and install a ball valve to several feet of heater hose. I mounted a couple of stainless spring clamps to route the 5/8" hose up to the engine hatch opening. Installed a garden hose fitting on the end of the hose and used a blanking plug to keep dirt out of the end.

Flushing process - Hook up garden hose, open flushing ball valve. LEAVE ENGINE WATER VALVE OPEN so as to not overpressure the impeller pump. This  will also backflush the strainer and water inlet. Turn on water and start engine. After flushing - Stop engine, turn of water, close flushing ball valve, close water inlet (hang key on handle), disconnect hose and install plug.  This process is very quick and easy to do before hosing off the boat.

Vinegar fill - unsnap hose from engine room wall and insert in gallon of white vinegar. Close engine ball valve and open flushing ball valve. Start  WARM engine and run until gallon container is empty. Stop engine, close flushing ball valve, hang key on inlet valve. I wouldn’t leave it in there for more than a week.

 I have already had to remove the manifold and drill out plugged passages in the block and manifold once and hope to avoid doing it again.)