watermark vs water line

watermark and water line both are nouns.

Word	Noun	Adjective	Verb	Adverb
watermark	Yes	No	No	No
water line	Yes	No	No	No

As nouns, watermark and water line are synonyms defined as:

watermark and water line: a line marking the level reached by a body of water

watermark (noun)	water line (noun)
a distinguishing mark impressed on paper during manufacture; visible when paper is held up to the light	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
a line marking the level reached by a body of water	a line marking the level reached by a body of water