sea robber vs plunderer

sea robber and plunderer both are nouns.

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

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

sea robber: 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 robber include despoiler, freebooter, looter, pillager, raider, spoiler.

sea robber (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)