I strongly recommend Salt-Away or the similar West Marine competing product. I've used them for several years. I wash down the boats, trailer/brakes, underside of towing vehicle/brakes and flush the motor/manifolds after every salt water use. Yes, pricey. Buy it in bulk to save. Run engine hot enough back home so the thermostat opens and flushes upper reaches of the engine cooling ports. On my Lancer I added a hose attachment flushing valve at the engine ahead of the water pump since the Volvo-Penta out-drive does not have a water impeller. For the Sundowner I have to use the muffs at the stern drive intakes. I think muffs feed is poor fit.
Also, as evidenced at my photo website,
http://netwinn205.shutterfly.com/ (blue heat exchanger in forground) I installed a closed loop cooling system to eliminate the salt water attack at the engine altogether. I still flush to benefit the manifolds which (unfortunately) are not part of the closed loop system. Found the closed loop cooling system here:
http://www.mrcool.us/ It may cost a few bucks but I think over the years it will save me money.