"ship's officer"

means a navigating officer or engineer officer, whether certificated or uncertificated, employed as such on board a ship, but does not include a master; and any reference to a ship's officer shall, in its application to a ship in which a mate, boatswain, marine engineman or assistant marine engineman is employed, be construed as including a reference to a mate, boatswain, marine engineman or assistant marine engineman;