red tide vs seawater

red tide and seawater both are nouns.

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

As nouns, seawater is a hypernym of red tide; that is, seawater is a word with a broader meaning than red tide:

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

Other hypernyms of red tide include brine, saltwater.

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