lighterman vs sea dog

lighterman and sea dog both are nouns.

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

As nouns, sea dog is a hypernym of lighterman; that is, sea dog is a word with a broader meaning than lighterman:

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

lighterman (noun)	sea dog (noun)
someone who operates a barge	a man who serves as a sailor