Plimsoll mark vs waterline

Plimsoll mark and waterline both are nouns.

Word	Noun	Adjective	Verb	Adverb
Plimsoll mark	Yes	No	No	No
waterline	Yes	No	No	No

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

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

Plimsoll mark (noun)	waterline (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