Under the engine cowling. That's why all boats with an enclosed engine are equipped with an exhaust fan that is supposed to be run for 2 minutes before starting the engine.
Without proper ventilation, the fumes are likely to accumulate in the bilges (the lowest part of a boat).
bilge
In the bilge.
Bilge
Bilge
Bilge
Bilge
bilge
Bilge
bilge
bilge
They are both heavier than air, and would accumulate in the lower parts of a boat (the bilges).