It won't be a problem unless you're in the salt, and if you're in salt, then you should be running heat exchangers. Using "raw water cooling" to me means, you're running a standard cooling system as opposed to a closed cooling system (the latter requires antifreeze and heat exchangers). Nobody should be running a standard cooling system in salt water (becuse it runs salt water through your engine block). That's fine with fresh lake or river water, by the way.
I saw this post come through and I agreed with Roy, but I didn't want to post bad info here on the forum (meaning, "my opinion") without talking to someone who would know for sure. I called George Gessford, at Gessford Machine http://www.gessford.com/
who was very gracious and took the call, and spent some time discussing the marine FE engines, and some of what he's doing.
He's just shipped 10 900-hp FE engines to New Zeland. To put it mildly, he knows what he's doing. He said "go with the FELPRO". He said unless you're doing something really crazy with your motor, you wouldn't need the specialty high power gaskets, O-rings, etc., that they use for the really big horsepower applications.
He said when he needs special gaskets, he gets them from Specialty Engine Components,
such as copper head gaskets, but he said they still put the FELPRO on top.
Bottom line, "use the FELPRO"