Another interpretation: They said, "and he took six hundred chariots" (Exodus 14:7); You too, "the chariots of Pharaoh and his host." They said, "and captains over all of them" (ibid.); You too, "the choicest of his captains were sunk in the Sea of Reeds." They embittered their lives with mortar; You too, by that same measure, did to them, for You made the waters like mortar, and they kept sinking into them. Therefore it is said, "sunk in the Sea of Reeds," and "sinking" refers to nothing but mortar, as it is said, "and Jeremiah sank into the mire" (Jeremiah 38:6), and it says, "I have sunk in deep mire" (Psalms 69:3). Therefore it is said, "sunk in the Sea of Reeds."
Six Hundred Chariots and the Drowning Mire of the Sea
Yalkut Shimoni on Torah 246:4
דָּבָר אַחֵר הֵם אָמְרוּ (שמות יד, ז) "וַיִּקַּח שֵׁשׁ מֵאוֹת רֶכֶב", אַף אַתָּה מַרְכְּבֹת פַּרְעֹה וְחֵילוֹ. הֵם אָמְרוּ (שם) "וְשָׁלִשִׁם עַל כֻּלּוֹ", אַף אַתָּה מִבְחַר שָׁלִשָׁיו טֻבְּעוּ בְיַם סוּף. הֵם מֵרְרוּ אֶת חַיֵּיהֶם בַּטִּיט, אַף אַתָּה בְּאוֹתָהּ מִדָּה עָשִׂיתָ לָהֶם מַיִם בַּטִּיט וְהָיוּ מִשְׁתַּקְּעִין בָּהֶם, לְכָךְ נֶאֱמַר טֻבְּעוּ בְיַם סוּף, וְאֵין טְבִיעָה אֶלָּא טִיט, שֶׁנֶּאֱמַר "וַיִּטְבַּע יִרְמְיָהוּ בַּטִּיט", וְאוֹמֵר (תהלים סט, ג) "טָבַעְתִּי בִּיוֵן מְצוּלָה", לְכָךְ נֶאֱמַר טֻבְּעוּ בְיַם סוּף.