Seeing as the water had already begun to crash down on the Egyptians, how could Yisrael be simultaneously walking on dry land?
Seforno: The water had begun to return at the point where both armies entered the sea, whereas Yisrael were at the other end.
Rashbam: What the Pasuk is saying is that, when the water crashed down on to the Egyptians, Yisrael had already passed through the dry sea-bed and were on the far bank. 1
Why does the Torah that Yisrael walked "on dry land in the middle of the sea", whereas above here (in Pasuk 22) it wrote "in the middle of the sea on dry land"?
Kol Eliyahu (Vilna Gaon): The earlier Pasuk is referring to the Tzadikim (like Nachshon ben Aminadav and the tribe of Yehudah), who entered the sea before it split; whilst the current Pasuk is speaking about the rest of Yisrael, who, lacking Emunah, would not enter until it turned into dry land.
And why does it write "Chomah" without a 'Vav'?
What is the significance of the wall of water "on their right and on their left"?
Da'as Zekeinim mi'Ba'alei Tosfos: The water on the right only turned into a wall when the angel Gavriel warned it not to warn Yisrael, who were destined to receive Torah from the right Hand of Hashem, and the water on the left, when he warned it not to harm Yisrael, who were destined to wear Tefilin on their left arm.