A sailor who serves on a submarine — someone whose job is living and working underwater for weeks or months at a time. It names the person, never the boat; the boat is a submarine. The word carries some respect in navy talk, because the life is famously hard: no daylight, tight space, long stretches with no news from home. You will hear it in war films, documentaries, and news reports about naval accidents. Stress is worth noting: British speakers usually say sub-MAR-in-er, while Americans often say SUB-marine-er, keeping the shape of 'submarine'. Both are understood. Outside naval contexts the word is rare, and it does not mean 'a diver' — a diver goes into the water; a submariner stays dry inside a vessel that goes into the water.