A crewman is one member of the crew on a boat or ship — someone who works there, not a passenger. You hear it in navy talk, older books, and films: “a crewman on the deck”, “every crewman knew the drill”. Though actually, the word points to a man. Today many people say crew member instead, because that covers everyone on the crew. On a ship you may still hear crewman; in careful or modern English, crew member is the safer choice. People do stretch it to planes and spacecraft (“a crewman on the shuttle”), but the home meaning here is still a person who works as part of a ship’s crew. It is a work word, not a fancy one — practical, a little old-fashioned.