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