water line vs Plimsoll

water line and Plimsoll both are nouns.

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

As nouns, Plimsoll is a hyponym of water line; that is, Plimsoll is a word with a more specific, narrower meaning than water line:

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
Plimsoll: waterlines to show the level the water should reach when the ship is properly loaded

Other hyponyms of water line include load line, Plimsoll line, Plimsoll mark.

water line (noun)	Plimsoll (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	waterlines to show the level the water should reach when the ship is properly loaded
a line marking the level reached by a body of water