Saltaway or Salt terminator flush (rinse boat also) after each use-installing a Perko flush-pro help with this. Replace raw water cooling fittings/elbows with brass. make sure you use anti-corrosive coatings on your engine etc when you do maintenance.
You'll want to fresh water rinse your riding gear and jackets after use also.
As mentioned above.. the trailer is the true weak link in the equation. Get a galvanized or aluminum one and rinse it well.
Closed cooling is not worth the expense-particularly on a block that is not brand new. Keep in mind that even in "closed" cooling systems the exhaust risers (the part most prone to fouling, rot, and failure) are still raw water cooled.
Salt phobia is pretty rampant in the ski/wake board community--something other boating communities chuckle over. There have been threads on this board that would lead a person to belive that if a ski boat touches salt water even once it will spontaneously corrode into a twisted heap and depreciate in value such that you will have to pay someone to take it off your hands.