saltwater vs red tide

saltwater and red tide both are nouns.

Word	Noun	Adjective	Verb	Adverb
saltwater	Yes	No	No	No
red tide	Yes	No	No	No

As nouns, red tide is a hyponym of saltwater; that is, red tide is a word with a more specific, narrower meaning than saltwater:

saltwater: water containing salts
red tide: seawater that is discolored by large numbers of certain dinoflagellates that produce saxitoxin

saltwater (noun)	red tide (noun)
water containing salts	seawater that is discolored by large numbers of certain dinoflagellates that produce saxitoxin