Sforno on Exodus
הלכו ביבשה, seeing the deep parts under the sea had all become frozen from the east wind so that the Israelites were crossing on frozen mud.
Or HaChaim on Exodus
בתוך הים ביבשה, in the midst of the sea on dry land. The words "in the midst of the sea" mean in the inner part of (what had formerly been) the sea; the word ביבשה means that the sea bed was not muddy but completely dry.