Importance of Fresh Air for Engine Compartment
The first legal requirement for boat ventilation was as far back as April 1940 when, by law, all boats powered by gasoline required sufficient ventilation. That law is still in place now.
If you move a few decades forward in time, the law was amended in 1982 to make a distinction between powered ventilation and natural ventilation. Every closed compartment containing a gas engine or gas tank must be ventilated by at least two ventilator ducts, and engine compartments containing a gas engine with a cranking motor must be ventilated by powered exhaust blowers operated from the instrument panel. This latter requirement applies to virtually all gas-powered inboard and sterndrive boats.
In most boats, there is always the risk, whether large or small, that gas fumes can build up. Without the right type of vents or accessories like blower fans, the buildup of fumes becomes dangerous. As well as posing health risks when inhaled, the fumes are also obviously very flammable and explosive.
Outboard-powered boats with permanent fuel tanks that are vented outside the boat are not required to have ventilation systems — unless they have a gas generator on board in an enclosed space. Diesel-powered inboards and sterndrives are also not subject to the ventilation requirement since diesel fumes are not explosive, but most still have systems to ventilate their engine compartments.