John Gill Commentary Psalms 104:9

"Thou hast set a bound that they may not pass over; That they turn not again to cover the earth." — Psalms 104:9 (ASV)

Thou hast set a bound, that they may not pass over

The Targum adds, ``to the rolling waves of the sea.'' Set doors with bolts and bars, cliffs, rocks, and shores: and, what is more surprising, sand, which is penetrable, flexible, and moveable, is set as a perpetual bound to the raging ocean and its waves, which they cannot pass over: see (Job 38:8–11) (Jeremiah 5:22) .

So the Lord has set a bound to the proud waters of afflictions, and says, Thus far shall you go, and no farther; and to the life of man, which he cannot exceed, (Job 14:5) . But he has given man a law, as a rule to walk by, as the boundary of his conversation, and this he transgresses; in which he is less tractable than the raging sea and its waves.

That they turn not again to cover the earth; as they did when it was first made, (Psalms 104:6) that is, not without the divine leave and power; for they did turn again and cover the earth, at the time of the flood; but never shall more. Some think there is no need to make this exception; since this was written after the flood, and when God had swore that the waters should no more go over the earth, (Isaiah 54:9) .