low-water mark vs hardship

low-water mark and hardship both are nouns.

Word	Noun	Adjective	Verb	Adverb
low-water mark	Yes	No	No	No
hardship	Yes	No	No	No

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

Other hypernyms of low-water mark include adversity, hard knocks.

low-water mark (noun)	hardship (noun)
a line marking the lowest level reached	something hard to endure
an extreme state of adversity; the lowest point of anything	something that causes or entails suffering
	a state of misfortune or affliction