boatswain vs sea dog

boatswain and sea dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
boatswain	Yes	No	No	No
sea dog	Yes	No	No	No

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

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

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

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