80 Heb “I will avenge your vengeance [= I will take vengeance for you; the phrase involves a verb and a cognate accusative].” The meaning of the phrase has been spelled out in more readily understandable terms.
81 Heb “I will dry up her [Babylon’s] sea and make her fountain dry.” “Their” has been substituted for “her” because “Babylonians” has been inserted in the previous clause and is easier to understand than the personification of Babylon = “her.” The reference to their sea is not clear. Most interpreters understand it to be a figurative reference to the rivers and canals surrounding Babylon. But some feel it refers to the reservoir that the wife of Nebuchadnezzar, Queen Nictoris, had made.

In other versions

Therefore thus says the Lord:

“Behold, I will plead your cause

and take vengeance for you.

I will dry up her sea

and make her fountain dry,

Therefore this is what the Lord says:

“See, I will defend your cause

and avenge you;

I will dry up her sea

and make her springs dry.

Therefore, this is what the Lord says:

I am about to champion your cause

and take vengeance on your behalf;

I will dry up her sea

and make her fountain run dry.

This is what the Lord says to Jerusalem:

“I will be your lawyer to plead your case,

and I will avenge you.

I will dry up her river,

as well as her springs,

Therefore thus says Yahweh,

“Look, I am going to contend your case,

and I will avenge your vengeance,

and I will cause her sea to dry up,

and I will cause her fountain to dry up.

Therefore thus saith the Lord;

Behold, I will plead thy cause,

And take vengeance for thee;

And I will dry up her sea,

And make her springs dry.

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