raider vs sea robber

raider and sea robber both are nouns.

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

As nouns, sea robber is a hyponym of raider; that is, sea robber is a word with a more specific, narrower meaning than raider:

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

Other hyponyms of raider include buccaneer, pirate, sea rover.

raider (noun)	sea robber (noun)
a corporate investor who intends to take over a company by buying a controlling interest in its stock and installing new management	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)