2 “And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear, and it was so.” And the water which was under the heaven gathered together unto one place; “And God called the dry land earth and the gathering together of the waters called He seas.” What trouble you have given me in my previous discourses by asking me why the earth was invisible, why all bodies are naturally endued with colour, and why all colour comes under the sense of sight.
And, perhaps, my reason did not appear sufficient to you, when I said that the earth, without being naturally invisible, was so to us, because of the mass of water that entirely covered it. Hear then how Scripture explains itself. “Let the waters be gathered together, and let the dry land appear.” The veil is lifted and allows the earth, hitherto invisible, to be seen. Perhaps you will ask me new questions. And first, is it not a law of nature that water flows downwards? Why, then, does Scripture refer this to the fiat of the Creator?
As long as water is spread over a level surface, it does not flow; it is immovable. But when it finds any slope, immediately the foremost portion falls, then the one that follows takes its place, and that one is itself replaced by a third. Thus incessantly they flow, pressing the one on the other, and the rapidity of their course is in proportion to the mass of water that is being carried, and the declivity down which it is borne. If such is the nature of water, it was supererogatory to command it to gather into one place.
It was bound, on account of its natural instability, to fall into the most hollow part of the earth and not to stop until the levelling of its surface. We see how there is nothing so level as the surface of water. Besides, they add, how did the waters receive an order to gather into one place, when we see several seas, separated from each other by the greatest distances? To the first question I reply: Since God's command, you know perfectly well the motion of water; you know that it is unsteady and unstable and falls naturally over declivities and into hollow places.
But what was its nature before this command made it take its course? You do not know yourself, and you have heard from no eye-witness. Think, in reality, that a word of God makes the nature, and that this order is for the creature a direction for its future course. There was only one creation of day and night, and since that moment they have incessantly succeeded each other and divided time into equal parts.
Source: Nine Homilies of Hexaemeron (New Advent)