Jack vs sea dog

Jack and sea dog both are nouns.

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

As nouns, Jack and sea dog are synonyms defined as:

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

Jack (noun)	sea dog (noun)
a man who serves as a sailor	a man who serves as a sailor