sailor vs sea dog

sailor and sea dog both are nouns.

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

As nouns, sea dog is a hyponym of sailor; that is, sea dog is a word with a more specific, narrower meaning than sailor:

sailor (noun)	sea dog (noun)
a stiff hat made of straw with a flat crown	a man who serves as a sailor
any member of a ship's crew
a serviceman in the navy