Was just looking into it and wondered if anyone did a closed system with headers. I totally understand the marine CAT systems plus need a bare min of 600 to 800 degree temps to operate for the reduction of emissions, The basic jist of that end.

Only thing I like about the header close cooling would be you will see a header leak when it happens.

If you flush your system directly after every use the half system will do the job nicely. The hard part when flushing is to get the salt our of the block and heads. The half system eliminates that problem. You will only take a few minutes per motor to flush the heat exchanger and headers. Another way to make it even easier and I've seen this on Nor-Techs is to plumb restricted dumps into the header manifolds to let them drain once the engine is shut down.

I thought about this with the cmi - 540 set up i am putting together.
Ran the same headers on a zz502, with partial external bypass so they already have partial chrome discoloration . Packed baffle silencers dry ,are very close in decibles to a wet exhaust and i personally think it just sounds better. If you could ramp up the header temps to a consistent 200 or so and go full dry exhaust with baffles it should make more power and not be obnoxiously loud.
If i ever get to put my personal stuff on the front burner this is something i wouldnt mind trying out

why not a 2 stage sea pump.
a seperate heat exchanger for the headers,
and an electric water pump that comes on when you turn the key