ship's officer vs old salt

ship's officer and old salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
ship's officer	Yes	No	No	No
old salt	Yes	No	No	No

As nouns, old salt is a hypernym of ship's officer; that is, old salt is a word with a broader meaning than ship's officer:

ship's officer: a person authorized to serve in a position of authority on a vessel
old salt: a man who serves as a sailor

Other hypernyms of ship's officer include gob, Jack-tar, Jack, mariner, sea dog, seafarer, seaman, tar.

ship's officer (noun)	old salt (noun)
a person authorized to serve in a position of authority on a vessel	a man who serves as a sailor