slack tide vs slack water

slack tide and slack water both are nouns.

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

As nouns, slack tide and slack water are synonyms defined as:

slack tide and slack water: the occurrence of relatively still water at the turn of the (low) tide

slack tide (noun)	slack water (noun)
the occurrence of relatively still water at the turn of the (low) tide	the occurrence of relatively still water at the turn of the (low) tide
	a stretch of water without current or movement