steersman vs sea dog

steersman and sea dog both are nouns.

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

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

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

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