פירוש על שמות 14:30

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).

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.

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.