The waters saw thee, O God! Some of the miracles in which God had displayed the power of his arm are here briefly adverted to. When it is said that the waters saw God, the language is figurative, implying that they were moved, as it were, by a secret instinct and impulse to obey the divine command in opening up a passage for the chosen people. Neither the sea nor the Jordan would have altered their nature, and by giving place have spontaneously afforded a passage to them, had they not both felt upon them the power of God. [1] It is not meant that they retired backward because of any judgment and understanding which they possessed, but that in receding as they did, God showed that even the inanimate elements are ready to yield obedience to him. There is here an indirect contrast, it being intended to rebuke the stupidity of men if they do not acknowledge in the redemption of the Israelites from Egypt the presence and hand of God, which were seen even by the waters. What is added concerning the deeps intimates, that not only the surface of the waters were agitated at the sight of God, but that his power penetrated even to the deepest gulfs.

Footnotes

1 - "The waters of the Red Sea,' says Bishop Horne, are here beautifully represented as endued with sensibility; as seeing, feeling, and being confounded, even to the lowest depths, at the presence and power of their great Creator, when he commanded them to open a way, and to form a wall on each side of it, until his people were passed over.' This, in fact, is true poetry; and in this attributing of life, spirit, feeling, action, and suffering, to inanimate objects, there are no poets who can vie with those of the Hebrew nation." -- Mant.

The waters saw thee - The waters of the Red Sea and the Jordan. There is great sublimity in this expression; in representing the waters as conscious of the presence of God, and as fleeing in consternation at his presence. Compare Revelation 20:11; Habakkuk 3:10-11.
They were afraid - On the word used here - חול chûl - see Psalm 10:5, note; Psalm 55:4, note. It may mean here to tremble or quake, as in pain Deuteronomy 2:25; Joel 2:6. - Alarm, distress, anguish, came over the waters at the presence of God; and they trembled, and fled.
The depths also were troubled - The deep waters, or the waters "in" the depths. It was not a ripple on the surface; but the very depths - the usually calm and undisturbed waters that lie below the surface - were heaved into commotion at the divine presence.

The waters saw thee - What a fine image! He represents God approaching the Red Sea; and the waters, seeing him, took fright, and ran off before him, dividing to the right and left to let him pass. I have not found any thing more majestic than this.
The depths also were troubled - Every thing appears here to have life and perception. The waters see the Almighty, do not wait his coming, but in terror flee away! The deeps, uncovered, are astonished at the circumstance; and as they cannot fly, they are filled with trouble and dismay. Under the hand of such a poet, inanimate nature springs into life; all thinks, speaks, acts; all is in motion, and the dismay is general.

The (k) waters saw thee, O God, the waters saw thee; they were afraid: the depths also were troubled.
(k) He declares how the power of God was declared when he delivered the Israelites through the Red Sea.

The waters saw thee, O God,.... The waters not of Jordan, but of the Red sea; these felt and perceived the power of God, in causing a strong east wind, which dried it up, and made way for the children of Israel to pass through it as on dry land: compare with this Psalm 114:3,
the waters saw thee; which is repeated for the confirmation of it, and to excite attention to it, as well as to express the psalmist's admiration at it; the Targum is,
"they saw thy majesty in the midst of the sea, O God; they saw thy power upon the sea;''
not the Egyptians, but the sons of Jacob and Joseph; the old Syriac church understood these waters of the waters of Jordan, at the baptism of Christ, when in their way they saw the incarnate God, and felt his sacred body laid in them, by which he was made manifest to Israel; but Jerom better interprets them, by the help of Revelation 17:15 of people, nations, and tongues; some of which saw Christ corporeally, others spiritually, and by faith, as preached in the Gospel to the Gentile world:
they were afraid; of the majesty of God, obeyed their Sovereign, of whom they stood in awe, gave way unto him, and fled at his rebuke, see Psalm 114:5 or "were in pain" (z), as a woman in travail, as were the Gentile world at the preaching of the Gospel of redemption and salvation by Christ, Romans 8:22,
the depths also were troubled; not only the surface, or waves of the waters, were moved by the strong east wind, through the power of God, but the bottom of the sea was reached by it; the depths were congealed in the midst of it, the channels of water were seen, and the foundation of the world discovered, and the children of Israel went through the deep as on dry land, see Exodus 15:8.
(z) "parturierunt", Montanus, Vatablus; "dolore corruptae sunt, videl dolore parturientium", Piscator; so Ainsworth.

When He directed His lance towards the Red Sea, which stood in the way of His redeemed, the waters immediately fell as it were into pangs of travail (יחילוּ, as in Habakkuk 3:10, not ויּחילו), also the billows of the deep trembled; for before the omnipotence of God the Redeemer, which creates a new thing in the midst of the old creation, the rules of the ordinary course of nature become unhinged. There now follow in Psalm 77:18, Psalm 77:19 lines taken from the picture of a thunder-storm. The poet wishes to describe how all the powers of nature became the servants of the majestic revelation of Jahve, when He executed judgment on Egypt and delivered Israel. זרם, Poel of זרם (cognate זרב, זרף, Aethiopic זנם, to rain), signifies intensively: to stream forth in full torrents. Instead of this line, Habakkuk, with a change of the letters of the primary passage, which is usual in Jeremiah more especially, has זרם מים עבר. The rumbling which the שׁחקים
(Note: We have indicated on Psalm 18:12; Psalm 36:6, that the שׁהקים are so called from their thinness, but passages like Psalm 18:12 and the one before us do not favour this idea. One would think that we have more likely to go back to Arab. sḥq, to be distant (whence suḥḳ, distance; saḥı̂ḳ, distant), and that שׁהקים signifies the distances, like שׁמים, the heights, from שׁחק = suḥḳ, in distinction from שׁחק, an atom (Wetzstein). But the Hebrew affords no trace of this verbal stem, whereas שׁחק, Arab. sḥq, contundere, comminuere (Neshwn: to pound to dust, used e.g., of the apothecary's drugs), is just as much Hebrew as Arabic. And the word is actually associated with this verb by the Arabic mind, inasmuch as Arab. saḥâbun saḥqun (nubes tenues, nubila tenuia) is explained by Arab. sḥâb rqı̂q. Accordingly שׁהקים, according to its primary notion, signifies that which spreads itself out thin and fine over a wide surface, and according to the usage of the language, in contrast with the thick and heavy פני הארץ, the uppermost stratum of the atmosphere, and then the clouds, as also Arab. a‛nân, and the collective ‛anan and ‛anân (vid., Isaiah, at Isaiah 4:5, note), is not first of all the clouds, but the surface of the sky that is turned to us (Fleischer).)
cause to sound forth (נתנוּ, cf. Psalm 68:34) is the thunder. The arrows of God (חצציך, in Habakkuk חצּיך) are the lightnings. The Hithpa. (instead of which Habakkuk has יחלּכוּ) depicts their busy darting hither and thither in the service of the omnipotence that sends them forth. It is open to question whether גּלגּל denotes the roll of the thunder (Aben-Ezra, Maurer, Bttcher): the sound of Thy thunder went rolling forth (cf. Psalm 29:4), - or the whirlwind accompanying the thunder-storm (Hitzig); the usage of the language (Psalm 83:14, also Ezekiel 10:13, Syriac golgolo) is in favour of the latter. On Psalm 77:19 cf. the echo in Psalm 97:4. Amidst such commotions in nature above and below Jahve strode along through the sea, and made a passage for His redeemed. His person and His working were invisible, but the result which attested His active presence was visible. He took His way through the sea, and cut His path (Chethb plural, שׁביליך, as in Jeremiah 18:15) through great waters (or, according to Habakkuk, caused His horses to go through), without the footprints (עקּבות with Dag. dirimens) of Him who passes and passed through being left behind to show it.

Afraid - And stood still, as men astonished, do.