sea-purse vs undercurrent

sea-purse and undercurrent both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea-purse	Yes	No	No	No
undercurrent	Yes	No	No	No

As nouns, undercurrent is a hypernym of sea-purse; that is, undercurrent is a word with a broader meaning than sea-purse:

sea-purse: the seaward undercurrent created after waves have broken on the shore
undercurrent: a current below the surface of a fluid

Other hypernyms of sea-purse include undertide.

sea-purse (noun)	undercurrent (noun)
the seaward undercurrent created after waves have broken on the shore	a subdued emotional quality underlying an utterance; implicit meaning
	a current below the surface of a fluid