slack water vs slack tide

slack water and slack tide both are nouns.

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

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

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

slack water (noun)	slack tide (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