Sforno on Exodus 14:29/ספורנו על שמות י״ד:כ״ט

1ובני ישראל הלכו ביבשה, all the while the Egyptians experienced all these problems and were drowning, the Israelites had been walking trough the sea as if it were dry land. The areas still being traversed by Israelites were not affected by the sea reverting to its originals state.