Charles Ellicott 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)

A boundary. —It is striking to observe what a deep impression their little line of coast, the barrier which repelled the waves of the Mediterranean, made on the Hebrew mind. The sea was an object of dread. Or if dread passed into reverent wonder, as in Psalm 104:25-26, it ended there; the Hebrew never took delight in the sea. Hence, the coast had for him only one purpose and suggestion. It was not for enjoyment or recreation, or even for uses of commerce. It was simply the defense set by God against the hostile waters.