all very nautical ol' Bean
Blame the Americans.
Juan Trippe of PanAm fame is generally given the credit for the uniform style and Captain First Officer designations which he took from the nautical professions. His aircraft were named with a "Clipper" prefix and used the name as their callsign, a reference to the "Clipper" ships of old which had been designed for speed. The aircraft were also referred to as "Ships", stemming from "Flying Boat". Prior to boarding all crew had to stand dockside in line at attention to be inspected by the Captain.