Psalms 46:3

Though the waters thereof roar and be troubled - The waters of the sea. The idea is, that they would not be afraid though everything should be in commotion, and be as unsettled as the restless waves of the ocean. The earth might be changed, the mountains removed, the agitated sea roar and dash against the shore, but their minds would be calm. The word rendered “be troubled” means to boil; to ferment; to foam; and here it refers to the ocean as agitated and lashed into foam. Nothing is more sublime and fearful than the ocean in a storm; nothing furnishes a better illustration of the peace produced by confidence in God amid the agitations which occur in the world, than the mind of a seaman that is calm when the ocean is heaved in wild commotion.

Though the mountains shake with the swelling thereof - The rolling ocean breaking against; the sides of the mountains on its shore, and seeming to shake them to their foundation. The word rendered “swelling” means properly majesty, glory; then pride, haughtiness, insolence. Literally, “though the mountains tremble through their pride.” Compare Psalms 124:5. On the word “Selah,” see the notes at Psalms 3:2.