Gas and propane fumes are most likely accumulate in the bilge and cabin floor area.
Bilge
Without proper ventilation, the fumes are likely to accumulate in the bilges (the lowest part of a boat).
Bilge
Bilge
In the bilge.
in the bilge
bilge
bilge
bilge
Moisture can accumulate in any headlight, most likely the cause is a bad seal between the two pieces of the fixture.
They are both heavier than air, and would accumulate in the lower parts of a boat (the bilges).