Exodus 15:5

יכסימו THEY COVER THEM — this is the same as יְכַסּוּם and the middle י is redundant. This is quite usual in Scripture–verses (in Biblical Hebrew), just as in (Deuteronomy 8:13) וצאנך ירביון, and (Psalms 36:9) ירויון מדשן ביתך, and the force of the first י which expresses the future tense you must explain as follows: they have been sunk in the Red Sea in order that the waters should return and should cover them. The word יכסימו has no similar example in Scripture so far as its punctuation is concerned; ordinarily it would be יְכַסְיֻמוּ with Melopum (חולם)‎. כמו אבן AS A STONE — But in another passage (v. 10) it states, “they dropped like lead”, and in yet another passage (v. 7) “it consumes them as stubble”! The wicked amongst the Egyptians were as stubble, being continually dashed about, and tossed up and down; those of average worth sank as a stone, suffering less agony, whilst the best amongst them sank as lead so that they came to their rest at once (cf. Mekhilta).

תְּהֹמֹ֖ת יְכַסְיֻ֑מוּ יָרְד֥וּ בִמְצוֹלֹ֖ת כְּמוֹ־אָֽבֶן׃        

The depths have covered them: they sank into the bottom like stone.