a)
n.
A dishwasher, or sometimes one who performs any kind of restaurant-based menial work.
Originating in 1920s France, "plongeur" is French for "diver," the idiom was presumably created as a result of the damp nature of the work, or possibly because it was a job largely performed in deep Parisian cellar-kitchens.
The
expression today is kept alive by smart-ass, American
Liberal Arts majors who think they're smart because they read Orwell, and want add an air of romance to cover up the shame of the fact that they'll be hosing down buffet plats in back of
the Golden Corral for at least eight years, until a cherry teaching job opens up, at a whopping forty grand a year.
b)
Some boring submarine no one cares about.