water line vs waterline

water line and waterline both are nouns.

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

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

water line and waterline: 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

Other synonyms of water line include water level.

water line (noun)	waterline (noun)
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 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