As a stone - The warriors in chariots are always represented on the monuments with heavy coats of mail; the corslets of "chosen captains" consisted of plates of highly tempered bronze, with sleeves reaching nearly to the elbow, covering the whole body and the thighs nearly to the knee. The wearers must have sunk at once like a stone, or as we read in Exodus 5:10, like lumps of lead.

The depths have covered them,.... The depths of the sea covered Pharaoh and his host, so as to be seen no more; and in like manner will mystical Babylon, or antichrist, be destroyed, and be no more found and seen; as likewise the sins of God's people, being cast into the depths of the sea, and covered with the blood of Christ, will be seen no more; when they are sought for, they shall not be found:
they sunk into the bottom as a stone; into the bottom of the sea, as a stone thrown into anybody of water sinks and rises not up again; this circumstance is observed by Nehemiah 9:11.
their persecutors thou threwest into the deep, as a stone into the mighty waters; and thus a stone like a millstone being taken by an angel and cast into the sea, is made an emblem of the irrecoverable ruin and destruction of Babylon, or antichrist, Revelation 18:21.

"Floods cover them (יכסימוּ, defectively written for יכסיוּ = יכסּוּ, and the suffix מוּ for מו, only used here); they go down into the deep like stone," which never appears again.