seawater vs red tide

seawater and red tide both are nouns.

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

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

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

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