sea rover vs plunderer

sea rover and plunderer both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea rover	Yes	No	No	No
plunderer	Yes	No	No	No

As nouns, plunderer is a hypernym of sea rover; that is, plunderer is a word with a broader meaning than sea rover:

sea rover: someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation
plunderer: someone who takes spoils or plunder (as in war)

Other hypernyms of sea rover include despoiler, freebooter, looter, pillager, raider, spoiler.

sea rover (noun)	plunderer (noun)
someone who robs at sea or plunders the land from the sea without having a commission from any sovereign nation	someone who takes spoils or plunder (as in war)