Diesel. Switch to diesel power.
MUCH more expensive to buy - MUCH more expensive to maintain plus not usually available in overlapping horsepower ranges with the big outboards. Overhauling a 400 hp diesel will run $50,000 - $90,000 depending on how extensive the overhaul, and when you consider they usually come in pairs it gets really outrageous. The Yahama 300 hp outboards run about $25,000 each for a brand new motor. I'm just winging it here on the price, but a 38 ftboat with twin diesels will probably cost about $175,000 more than a 38 footer with triple outboards.
I read the other day where the diesel that they tested in LA had cleaner exhaust than the air that went in the carb!