It's ENGINEERED STUPIDITY!
You can't take a complete combustion engine, a battery (or 3), and 40-60 gallons of fuel, and cram it all into the bilge area of a boat......then bury it all tightly under upholstery, with a piddly venting system......and expect it not to eventually explode.
Especially in a craft that sits for months at a time with anything less than constant impeccable maintenance.
Noone short of a qualified marine technician has any business owning/operating a I/O