sea creature vs beast

sea creature and beast both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea creature	Yes	No	No	No
beast	Yes	No	No	No

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

sea creature: any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals
beast: a living organism characterized by voluntary movement

Other hypernyms of sea creature include animal, animate being, brute, creature, fauna.

sea creature (noun)	beast (noun)
any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals	a living organism characterized by voluntary movement
	a cruelly rapacious person