Here is a theory that can be easily tested to see if this is the cause for all the salt
encrustation inside your unfortunate engine:
Just as glass jars are 'vacuum sealed', where the lid is puckered down by placing the lid while the jar is hot and as it cools, the lid is pulled down with a natural vacuum, a similar thing is still happening to your engine. I'll explain.....
When your engine is shut down, it is hot, the exhaust manifold and pipe to the mixing elbow
are red hot. As everything cools, it forms a natural vacuum sucking sea water through those 2 exhaust hoses, 'straws' , that are filled with the cooling
water and a little water gets into the exhaust manifold each time the engine cools down.
Nah, it's not the metal gasket
at the manifold that was leaking deadly carbon monoxide, smelly exhaust, smoke and soot as well as sea water because you would have smelled it, seen the smoke in the engine room and been able to see the trace of the salt water
leak or rusty streaks down the side.
Don't let the engine get damaged again based on an assumption, try this easy test:
Remove those 2 hoses from the manifold when the engine is off and cool, slowly fill with water as if to fill the muffler and see how far up you can still hold the solid water level on each. Probably close to the level of the exhaust elbow
because the exit at the exhaust on one of them is higher than the elbow, and that's fine, but it needs to drain quickly down into the muffler and then to have a sudden upward rise of the hose out of the muffler and have a vacuum breaker, a vent, atop that loop so that the hoses would never have solid water filling the inside of them as to not be able to suck any back out, not even from inside the muffler.
Some of that water you see inside the hose is being sucked into the exhaust manifold by the natural vacuum of the cooling
engine and when cool, the salt
is precipitated and cakes onto the insides of the exhaust box and adjacent #4 cylinder.
The open or closed valves on the engine do not open the exhaust manifold to the atmosphere, a leaking metal gasket
at the elbow would allow that and break the natural vacuum but that's deadly.
This is only a suggestion, an easy test and a theory, but to be sure, do revise the hoses and the exhaust system to minimize any possibility of sitting, solid water being available so close to the exhaust. Make it impossible for the natural vacuum to bring sea water into the manifold, even if by increasing the capacity of the hoses with a larger diameter soon after the elbow and then reduce it again if you must before the muffler. The water level test will give you an idea.
We are all hoping that your engine if fixed and stays fixed. Better New Year to you.