וַיּ֨וֹשַׁע יְהוָ֜ה בַּיּ֥וֹם הַה֛וּא אֶת־יִשְׂרָאֵ֖ל מִיַּ֣ד מִצְרָ֑יִם וַיַּ֤רְא יִשְׂרָאֵל֙ אֶת־מִצְרַ֔יִם מֵ֖ת עַל־שְׂפַ֥ת הַיָּֽם׃
Thus the LORD saved Israel that day out of the hand of the Egyptians; and Israel saw the Egyptians dead upon the sea-shore.
Rashi on Exodus
וירא ישראל את מצרים מת AND ISRAEL SAW THE EGYPTIANS DEAD — because the sea threw them out on its shore in order that the Israelites should not say: “Just as we have come up from out of the sea on this part, so they have come up on another part of this shore, but far away from us, and they will pursue us” (Mekhilta d'Rabbi Yishmael 14:31:1; Pesachim 118b; Arakhin 15a).
Sforno on Exodus
ויושע ה' ביום ההוא את ישראל, by the death of those who had cruelly abused them. Those who had previously been in bondage to the Egyptians were now free. Until the Egyptians died they had been comparable to slaves running away from their masters.
Or HaChaim on Exodus
ביום ההוא, on that day, etc. The Torah simply means that the salvation only occurred on that day, not previously; even though the Exodus had occurred, the Israelites had not yet acquired much confidence in themselves vis-a-vis the Egyptians.