low-water mark vs hard knocks

low-water mark and hard knocks both are nouns.

Word	Noun	Adjective	Verb	Adverb
low-water mark	Yes	No	No	No
hard knocks	Yes	No	No	No

As nouns, hard knocks is a hypernym of low-water mark; that is, hard knocks is a word with a broader meaning than low-water mark:

Other hypernyms of low-water mark include adversity, hardship.

low-water mark (noun)	hard knocks (noun)
a line marking the lowest level reached	a state of misfortune or affliction
an extreme state of adversity; the lowest point of anything