3 “Let the waters be gathered together.” It was ordered that it should be the natural property of water to flow, and in obedience to this order, the waters are never weary in their course. In speaking thus, I have only in view the flowing property of waters. Some flow of their own accord like springs and rivers, others are collected and stationary. But I speak now of flowing waters. “Let the waters be gathered together unto one place.” Have you never thought, when standing near a spring which is sending forth water abundantly, Who makes this water spring from the bowels of the earth? Who forced it up? Where are the store-houses which send it forth? To what place is it hastening? How is it that it is never exhausted here, and never overflows there? All this comes from that first command; it was for the waters a signal for their course.
In all the story of the waters remember this first order, “let the waters be gathered together.” To take their assigned places they were obliged to flow, and, once arrived there, to remain in their place and not to go farther. Thus in the language of Ecclesiastes, “All the waters run into the sea; yet the sea is not full.” Waters flow in virtue of God's order, and the sea is enclosed in limits according to this first law, “Let the waters be gathered together unto one place.” For fear the water should spread beyond its bed, and in its successive invasions cover one by one all countries, and end by flooding the whole earth, it received the order to gather unto one place. Thus we often see the furious sea raising mighty waves to the heaven, and, when once it has touched the shore, break its impetuosity in foam and retire. “Fear ye not me, says the Lord....which have placed the sand for the bound of the sea.” A grain of sand, the weakest thing possible, curbs the violence of the ocean. For what would prevent the Red Sea from invading the whole of Egypt, which lies lower, and uniting itself to the other sea which bathes its shores, were it not fettered by the fiat of the Creator? And if I say that Egypt is lower than the Red Sea, it is because experience has convinced us of it every time that an attempt has been made to join the sea of Egypt to the Indian Ocean, of which the Red Sea is a part. Thus we have renounced this enterprise, as also have the Egyptian Sesostris, who conceived the idea, and Darius the Mede who afterwards wished to carry it out.
I report this fact to make you understand the full force of the command, “Let the waters be gathered unto one place”; that is to say, let there be no other gathering, and, once gathered, let them not disperse.
Source: Nine Homilies of Hexaemeron (New Advent)