Chev 350 vs Ford 351 for salt water?

I have heard that the PCM Ford 351 Windsor marine block, if there is a special marine block, will last much longer with salt water running through it than a PCM 350 Chevy block will last. Is that true? Did/does Ford make a 351 marine block different from their standard 351?

I have run my 1994 Ski Nautique with the 351 exclusively in salt water flushing it with fresh water only at the beginning of the season in May and again when I take it out in October and the block is doing very well so far. I have installed new exhaust manifolds twice since 1994 just as a precaution. I'm wondering how many more seasons I can expect out of my '94 block before it rusts through.

I knew I was going to hang this boat on a buoy in front of our Puget Sound home in a situation where it would be nearly impossible to flush it out after each use SO I asked a lot of questions of a lot of Puget Sound boaters and I learned that while some people did spring for fresh water cooling the vast majority of inboard boats which lived in the salt water year round ran salt water through their cast iron blocks and heads and never flushed them ever. I found that hard to believe at first but became convinced the more I looked into it. I concluded that if all of these yachts' engines survived mine would too so I went ahead and purchased our Nautique and have had nearly zero problems ever since.

Is there such a thing as a "Marine" Ford 351 Windsor or a "Marine" 350 Chevy block which I'm assuming would be thicker in the water jacket areas?

Do 351 Ws have a reputation of lasting longer in salt water than the 350s? I'm pretty sure the aluminum heads, which I think come on the GT 40s are a bad idea in salt water.

I know South Florida waters are tough and seem to be way worse than northern waters. Dads PCM powered boat was on it's 3rd set of exhausts in 20 years,along with 1 upper thermostat housing. Never spent the night in the water and flushed after each use,in use from November to April.
Here is a picture of a PCM Ford intake manifold in use from 78-93 in south florida when we bought it. No idea on flushing but I'm guessing not often. I went to reuse this but decided against it since there not much sealing surface left. This is looking into the water passage with the temp boss in the back ground. I think Ford blocks will be available due to the shear number of them being made. What will be troublesome is intakes,thermostat housings,timing covers etc.