Definition
Verb
- “In” refers to being a member of a ship’s crew.
Origin
19th century.
Comments
The distinction being that “in a ship” refers to a person who is a member of the crew, versus “on a ship” which is being a passenger (non-crew member). Apparently, the semantic difference is mostly for purists (or the British).
References
Rogers, J.G. (1985). Origins of Sea Terms. Mystic Seaport Museum.
Fair Winds & Following Seas 
I recollect hearing people use “of’ rather than “in” as in referring to someone being “of a ship’s company “( crew).
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