saltwater fish vs seafood

saltwater fish and seafood both are nouns.

Word	Noun	Adjective	Verb	Adverb
saltwater fish	Yes	No	No	No
seafood	Yes	No	No	No

As nouns, seafood is a hypernym of saltwater fish; that is, seafood is a word with a broader meaning than saltwater fish:

saltwater fish: flesh of fish from the sea used as food
seafood: edible fish (broadly including freshwater fish) or shellfish or roe etc

saltwater fish (noun)	seafood (noun)
flesh of fish from the sea used as food	edible fish (broadly including freshwater fish) or shellfish or roe etc