3 But then other citizens of the holy Jerusalem, understanding their captivity, mark how the natural wishes and the various lusts of men hurry and drag them hither and there, and drive them into the sea; they see this, and they throw not themselves into the waters of Babylon, but “sit down and weep,” either for those who are being carried away by them, or themselves whose deserts have placed them in Babylon, but sitting, that is, humbling themselves. O holy Sion, where all stands firm and nothing flows!
Who has thrown us headlong into this? Why have we left your Founder and your society? Behold, placed where all things are flowing and gliding away, scarce one, if he can grasp the tree, shall be snatched from the stream and escape. Humbling ourselves then in our captivity, let us “sit by the waters of Babylon,” let us not dare to plunge ourselves in those streams, nor to be proud and lifted up in the evil and sadness of our captivity, but let us sit, and so weep. Let us sit “by” the waters, not beneath the waters, of Babylon; such be our humility, that it overwhelm us not. Sit “by” the waters, not “in” the waters, not “under” the waters; but yet sit, in humble fashion, talk not as you would in Jerusalem....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)