Plimsoll vs water line

Plimsoll and water line both are nouns.

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

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

Plimsoll: waterlines to show the level the water should reach when the ship is properly loaded
water line: 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 hypernyms of Plimsoll include water level, waterline.

Plimsoll (noun)	water line (noun)
waterlines to show the level the water should reach when the ship is properly loaded	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