9 Wherefore what has He done by the terror of His counsel? He has turned the sea into dry land. For this follows, “That has turned the sea into dry land”. A sea was the world, bitter with saltness, troubled with tempest, raging with waves of persecutions, sea it was: truly into dry land the sea has been turned, now there thirsts for sweet water the world that with salt water was filled. Who has done this? He “that has turned the sea into dry land.” Now the soul of all the Gentiles says what?
“My soul is as it were land without water to You.” “That has turned the sea into dry land. In the river they shall pass over on foot.” Those same persons that have been turned into dry land, though they were before sea, “in the river on foot shall pass over.” What is the river? The river is all the mortality of the world. Observe a river: some things come and pass by, other things that are to pass by do succeed. Is it not thus with the water of a river, that from earth springs and flows?
Every one that is born must needs give place to one going to be born: and all this order of things rolling along is a kind of river. Into this river let not the soul greedily throw herself, let her not throw herself, but let her stand still. And how shall she pass over the pleasures of things doomed to perish? Let her believe in Christ, and she will pass over on foot: she passes over with Him for Leader, on foot she passes over.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)