low-water mark vs water line

low-water mark and water line both are nouns.

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

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

Other hypernyms of low-water mark include watermark.

low-water mark (noun)	water line (noun)
a line marking the lowest level reached	a line corresponding to the surface of the water when the vessel is afloat on an even keel; often painted on the hull of a ship
an extreme state of adversity; the lowest point of anything	a line marking the level reached by a body of water