According to the Centers for Disease Control, Fresh Air Exhaust reduces Carbon Monoxide and other harmful exhaust gasses by at least 90%.
Unless you boat is equipped with a catalytic converter, it produces lethal levels of Carbon Monoxide (greater than 10,000 ppm); levels strong enough to cause you to lose consciousness within about 3 minutes. Sadly, deaths from drowning occur. The little girl in this picture was sitting on the swim platform of the boat as it was traveling in a Fourth of July parade. She was non-responsive when her father realized she had moved to the swim platform. She spent several hours on oxygen in a hyperbaric chamber.
Even on boats with catalytic converters, the Centers for Disease Control recommends underwater exhaust (FAE) to further reduce Carbon Monoxide levels. Without underwater exhaust you are still breathing the toxic fumes from a V8 gasoline engine.