The Lord thundered from heaven, and the most High uttered his voice. See Gill on Psalm 18:13.

14 Jehovah thundered from the heavens,
And the Most High gave His voice.
15 He sent arrows, and scattered them;
Lightning, and discomfited them.
16 Then the beds of the sea became visible;
The foundations of the world were uncovered,
Through the threatening of Jehovah,
By the snorting of the breath of His nostrils.
God sent lightning as arrows upon the enemies along with violent thunder, and threw them thereby into confusion. המם, to throw into confusion, and thereby to destroy, is the standing expression for the destruction of the foe accomplished by the miraculous interposition of God (vid., Exodus 14:24; Exodus 23:27; Joshua 10:10; Judges 4:15; 1-Samuel 7:10). To the thunder there were added stormy wind and earthquake, as an effect of the wrath of God, whereby the foundations of the sea and land were laid bare, i.e., whereby the depth of the abyss and of the hell in the interior of the earth, into which the person to be rescued had fallen, were disclosed.
(Note: In 2-Samuel 22:13-16 the text of the Psalm deviates greatly and in many instances from that before us. In v. 13 we find אשׁ וגחלי בּרד עברוּ עביו instead of אשׁ גּחלי בּערוּ; and after v. 14 אשׁ וגחלי בּרד is repeated in the psalm. In v. 15 we have רב וּברקים for בּרק, and in v. 16 מים אפיקי for ים אפיקי. The other deviations are inconsiderable. So far as the repetition of אשׁ וגחלי בּרד at the end of v. 14 is concerned, it is not only superfluous, but unsuitable, because the lightning following the thunder is described in v. 15, and the words repeated are probably nothing more than a gloss that has crept by an oversight into the text. The מים אפיקי in v. 16 is an obvious softening down of the ים אפיקי of the text before us. In the other deviations, however, the text of the Psalm is evidently the more original of the two; the abridgment of the second clause of v. 13 is evidently a simplification of the figurative description in the psalm, and רב בּרקים in the 15th verse of the psalm is more poetical and a stronger expression than the mere בּרק of our text.)