"The deeps covered them" (Exodus 15:5). But are there deeps there? Is it not a shallow place? Rather, this teaches that the upper deep and the lower deep rose up, and the waters fought against them with every kind of punishment. Therefore it is said, "the deeps covered them."
The Deeps Cover Them as Upper and Lower Waters Make War
Yalkut Shimoni on Torah 246:5
(שמות טו ה-ו) תְּהֹמֹת יְכַסְיֻמוּ. וְכִי תְּהוֹמוֹת יֵשׁ שָׁם, וַהֲלֹא עֲשׁוֹנִית הִיא, אֶלָּא מְלַמֵּד שֶׁעָלוּ תְּהוֹם הָעֶלְיוֹן וּתְהוֹם הַתַּחְתּוֹן וְהָיוּ הַמַּיִם נִלְחָמִים בָּהֶם בְּכָל מִיּנֵי פֻּרְעָנֻיּוֹת, לְכָךְ נֶאֱמַר תְּהֹמֹת יְכַסְיֻמוּ.