bosun vs old salt

bosun and old salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
bosun	Yes	No	No	No
old salt	Yes	No	No	No

As nouns, old salt is a hypernym of bosun; that is, old salt is a word with a broader meaning than bosun:

bosun: a petty officer on a merchant ship who controls the work of other seamen
old salt: a man who serves as a sailor

Other hypernyms of bosun include gob, Jack-tar, Jack, mariner, sea dog, seafarer, seaman, tar.

bosun (noun)	old salt (noun)
a petty officer on a merchant ship who controls the work of other seamen	a man who serves as a sailor