Welcome to the forum.
I know that Air Seps are the cats meow for Detroit Diesels. They control blowby venting and all of the fumes, soot and oil is filtered, collected and returned to the crank case.
But can you make the case for a typical 4 cycle engine used in most older trawlers, like the Lehman or Perkins. My buddy has a couple on his trawler and I don't see any fumes, soot or oil in the vent.
Even less so for a more modern engine like the Yanmar 6LY. I get a drop of oil out of the blowby tube every 10 hours or so. I just wash out the air filter element every 100 hours or so and all is good.