seaman vs old salt

seaman and old salt both are nouns.

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

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

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

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