Crewman is a skill, described in Basic Set, p. 185. There are separate skills for different classes of vessel: Airshipman, Seamanship, Spacer, and Submarine. GURPS Low Tech™, p. 10, adds the new version Oarsman. Seamanship is referenced in GURPS Dungeon Fantasy 2: Dungeons™, p. 5, on getting to a dungeon. Spacer is referenced in GURPS Space, p. 226, on its appropriateness as a background skill.