boatswain vs old salt

boatswain and old salt both are nouns.

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

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

boatswain: 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 boatswain include gob, Jack-tar, Jack, mariner, sea dog, seafarer, seaman, tar.

boatswain (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