Well, I'm no expert on fresh vs salt and corrosion. But my personal preference is to avoid raw water cooling. Closed, or fresh water cooling (FWC) as sometimes called, circulates just antifreeze throughout the engine block, not water straight from below the boat.
What I have heard raw water folks adding is a flush system. Something that lets you force fresh water through the system after you're done using it. Usually done with a hose from shore water when you're back at the marina and done using the boat for a few days. Better to leave tap water in there instead of salt water.
Then there's the question of how old are your current risers and manifolds? Some fresh water boaters have gotten decades out of theirs. But if yours are on their last legs that'll make it a bit less painful to replace the whole system "while you're at it". Of course you can't TELL whether the risers need it unless you remove them an physically inspect them.
My suggestion would be to get estimates on replacing them as-is, adding a flush system and switching to FWC. Worst case you run the existing ones until they die (running some risk of engine problems), stretch their life a bit longer by flushing them, or biting the bullet and making the upgrade. With estimates you'll at least know the eventual costs.
_________________ -Bill Kearney, 2005 348 Vista
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