ship's officer vs sea dog

ship's officer and sea dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
ship's officer	Yes	No	No	No
sea dog	Yes	No	No	No

As nouns, sea dog is a hypernym of ship's officer; that is, sea dog is a word with a broader meaning than ship's officer:

ship's officer: a person authorized to serve in a position of authority on a vessel
sea dog: a man who serves as a sailor

Other hypernyms of ship's officer include gob, Jack-tar, Jack, mariner, old salt, seafarer, seaman, tar.

ship's officer (noun)	sea dog (noun)
a person authorized to serve in a position of authority on a vessel	a man who serves as a sailor