He maketh the storm a calm; i.e. he causes the wind to drop, and to be succeeded by a "great calm" (comp. Matthew 8:26). Such sudden changes do sometimes occur, not only in inland seas, but on the Mediterranean (see Jonah 1:15).
So that the waves thereof are still; literally, the waves of them; i.e. of the great waters (see Psalms 107:23).