Another interpretation: Alef is one, zayin is seven; behold, eight [the word az being spelled alef-zayin]. Moses said: By the merit of circumcision, which was given for the eighth day, the sea was split; with "then" (az) let us praise. Rabbi Levi said: "To Him who divided the Sea of Reeds into parts (li-gezarim)" (Psalms 136:13) - for so in the Aramaic tongue they call the circumcised "the cut ones" (gezurin). By the merit of circumcision the sea was split.
The Sea Was Split by the Merit of Circumcision on the Eighth Day
Yalkut Shimoni on Torah 241:4
דָּבָר אַחֵר א' אֶחָד ז' שִׁבְעָה הֲרֵי שְׁמוֹנָה, אָמַר מֹשֶׁה בִּזְכוּת הַמִּילָה שֶׁנִתְּנָה לִשְׁמוֹנָה נִקְרַע הַיָּם, בְּאָז נְקַלֵּס. אָמַר רַבִּי לֵוִי, "לְגֹזֵר יַם סוּף לִגְזָרִים", שֶׁכֵּן בִּלְשׁוֹן אֲרַמִּי קוֹרִין לִמְהוּלִין גְּזוּרִין, בִּזְכוּת הַמִּילָה נִקְרַע הַיָּם.