low-water mark vs nadir

low-water mark and nadir both are nouns.

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

As nouns, low-water mark and nadir are synonyms defined as:

low-water mark and nadir: an extreme state of adversity; the lowest point of anything

low-water mark (noun)	nadir (noun)
a line marking the lowest level reached	the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected
an extreme state of adversity; the lowest point of anything	an extreme state of adversity; the lowest point of anything