Another interpretation of "the chariots of Pharaoh" and so on: By the very measure with which they measured, You measured out to them. They had said, "Every son that is born you shall cast into the Nile" (Exodus 1:22) and so on; You too measured out to them by that same measure. Therefore it is said, "the chariots of Pharaoh and his host He has cast into the sea."
Measure for Measure as Pharaoh's Chariots Are Cast into the Sea
Yalkut Shimoni on Torah 246:3
דָּבָר אַחֵר מַרְכְּבֹת וְגוֹ', בְּמִדָּה שֶׁמָּדְדוּ בָּהּ מָדַדְתָּ לָהֶם. הֵן אָמְרוּ (שמות א, כב) "כָּל הַבֵּן הַיִּלּוֹד הַיְּאֹרָה תַּשְׁלִיכֻהוּ" וְגוֹ', אַף אַתָּה בְּאוֹתָהּ מִדָּה מָדַדְתָּ לָהֶם, לְכָךְ נֶאֱמַר מַרְכְּבֹת פַּרְעֹה וְחֵילוֹ יָרָה בַיָּם.