old salt vs seaman

old salt and seaman both are nouns.

Word	Noun	Adjective	Verb	Adverb
old salt	Yes	No	No	No
seaman	Yes	No	No	No

As nouns, old salt and seaman are synonyms defined as:

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

old salt (noun)	seaman (noun)
a man who serves as a sailor	a man who serves as a sailor