12 Now therefore, O Asaph, amend yourself according to your understanding, tell us what sort of Salvation God has wrought in the midst of the earth. When that earthly Salvation of yours was overthrown, what did He do, what did He promise? “You confirmed in Your virtue the sea”. As though the nation of the Jews were as it were dry land severed from the waves, the Gentiles in their bitterness were the sea, and on all sides they washed about that land: behold, “You have confirmed in Your virtue the sea,” and the land remained thirsting for Your rain. “You have confirmed in Your virtue the sea, You have broken in pieces the heads of dragons in the water.” Dragons' heads, that is, demons' pride, wherewith the Gentiles were possessed, You have broken in pieces upon the water: for those persons whom they were possessing, You by Baptism have delivered.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)