boatswain vs mariner

boatswain and mariner both are nouns.

Word	Noun	Adjective	Verb	Adverb
boatswain	Yes	No	No	No
mariner	Yes	No	No	No

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

boatswain: a petty officer on a merchant ship who controls the work of other seamen
mariner: a man who serves as a sailor

Other hypernyms of boatswain include gob, Jack-tar, Jack, old salt, sea dog, seafarer, seaman, tar.

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