low-water mark vs watermark

low-water mark and watermark both are nouns.

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

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

Other hypernyms of low-water mark include water line.

low-water mark (noun)	watermark (noun)
a line marking the lowest level reached	a distinguishing mark impressed on paper during manufacture; visible when paper is held up to the light
an extreme state of adversity; the lowest point of anything	a line marking the level reached by a body of water