slack water vs tide

slack water and tide both are nouns.

Word	Noun	Adjective	Verb	Adverb
slack water	Yes	No	No	No
tide	Yes	No	Yes	No

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

slack water: the occurrence of relatively still water at the turn of the (low) tide
tide: the periodic rise and fall of the sea level under the gravitational pull of the moon

slack water (noun)	tide (noun)
the occurrence of relatively still water at the turn of the (low) tide	the periodic rise and fall of the sea level under the gravitational pull of the moon
a stretch of water without current or movement	something that may increase or decrease (like the tides of the sea)
	there are usually two high and two low tides each day