tide vs slack water

tide and slack water both are nouns.

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

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

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

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