sea dog vs Jack

sea dog and Jack both are nouns.

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

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

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

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