ebb vs hem in

ebb is a noun but hem in is not a noun.

Word	Noun	Adjective	Verb	Adverb
ebb	Yes	No	Yes	No
hem in	No	No	Yes	No

As verbs, hem in is a hypernym of ebb; that is, hem in is a word with a broader meaning than ebb:

ebb: hem in fish with stakes and nets so as to prevent them from going back into the sea with the ebb
hem in: surround so as to force to give up

Other hypernyms of ebb include beleaguer, besiege, circumvent, surround.

ebb (noun)	hem in (noun)
the outward flow of the tide
a gradual decline (in size or strength or power or number)

ebb (verb)	hem in (verb)
fall away or decline	surround so as to force to give up
hem in fish with stakes and nets so as to prevent them from going back into the sea with the ebb	surround in a restrictive manner
flow back or recede