sea dog vs old salt

sea dog and old salt both are nouns.

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

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

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

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