sea-purse vs undertide

sea-purse and undertide both are nouns.

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

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

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

Other hypernyms of sea-purse include undercurrent.

sea-purse (noun)	undertide (noun)
the seaward undercurrent created after waves have broken on the shore	a current below the surface of a fluid