old salt vs sea dog

old salt and sea dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
old salt	Yes	No	No	No
sea dog	Yes	No	No	No

As nouns, old salt and sea dog are synonyms defined as:

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

old salt (noun)	sea dog (noun)
a man who serves as a sailor	a man who serves as a sailor