helmsman vs sea dog

helmsman and sea dog both are nouns.

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

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

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

helmsman (noun)	sea dog (noun)
the person who steers a ship	a man who serves as a sailor