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-purse, sea-puss, 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