undertide vs sea-purse

undertide and sea-purse both are nouns.

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

As nouns, sea-purse is a hyponym of undertide; that is, sea-purse is a word with a more specific, narrower meaning than undertide:

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

Other hyponyms of undertide include sea-poose, sea-puss, sea purse, sea puss, undertow.

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