17 And he continues how? “The waters have seen You, O God, and they have feared and the abysses have been troubled”. What are the waters? The peoples. What are these waters has been asked in the Apocalypse, the answer was, the peoples. There we find most clearly waters put by a figure for peoples. But above he had said, “You have made known in the peoples Your virtue.” With reason therefore, “the waters have seen You, and they have feared.” They have been changed because they have feared. What are the abysses? The depths of waters. What man among the peoples is not troubled, when the conscience is smitten? You seek the depth of the sea, what is deeper than human conscience? That is the depth which was troubled, when God redeemed with His arm His people. In what manner were the abysses troubled? When all men poured forth their consciences in confession.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)