I've surveyed dozens of boat with A4's and don't understand why they have not all blown up ! Every one I have seen has at least 2 or 3 of the following issues.
1.) Improperly installed engine compartment ventilation systems (per ABYC).
2.) Engine compartment not vapour proof to the accomodation spaces.
3.) Non-ignition proof battery (auto type)chargers in the fuel/engine
4.) AC outlets in the fuel/engine compartment.
5.) Ungrounded fuel fill/tank.
Actual selling price of a diesel vs. gas in this category is usually 2-4k more.
Ahhhh ! diesel fumes ....mothers milk