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
The Torah said above (Pasuk 22) that Bnei Yisrael went through the sea. Why was this repeated? And above it wrote "in the middle of the sea on dry land", and here, it said "on dry land in the middle of the sea"! And above, "Chomah" is written with a Vav, and here, without a 'Vav'!
Kol Eliyahu, Divrei Eliyahu, Lev Eliyahu p.82: Pasuk 22 discusses Ba'alei Emnah, like Nachshon ben Aminadav and the tribe of Yehudah, who entered the sea before it split. Here it discusses those without such Emunah, would entered only after it turned into dry land. Chomah without a 'Vav' reads 'Cheimah' (anger). The sea complained - also these served idolatry. Why should they be saved, and the Egyptians drown?
Kol Eliyahu: Pasuk 22 discusses when the first Shevatim entered the sea, and others were still on the dry land. Here it discusses when the first Shevatim came out onto the dry land, and the last tribe, Dan, 1 was in the sea. Michah, from Dan, crossed with the idol 2 that he later set up in Eretz Yisrael. Chomah without a 'Vav' reads 'Cheimah' (anger). There was prosecution - why should a miracle be done when an abomination is with them?!
Why does it say "they went on the dry land? The tribes that came out did not need to go anywhere! Perhaps they had to distance from the shore to make room for others to alight. If there was a lane for each Shevet (refer to 4:21:4:1), all could leave the sea together! It seems that Dan was chosen to be the last Shevet only when Hashem fixed the Degalim, in Parshas Bamidbar! Perhaps Dan was last to enter, therefore they were chosen to travel last. (PF)
Ba'al ha'Turim: The final letters of "v'Avar ba'Yam Tzarah", Ramah, have the same Gematriya as "Pesel Michah." Roke'ach (Perush Az Yashir) - Michah did not cross with the idol. Rather, he took coins needed for it
What is the significance of the wall of water "on their right and on their left"?
Da'as Zekeinim, Hadar Zekenim (22): 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, and in back to be careful about Yisrael, who will put on Talesim in back. 1
Hadar Zekenim (22): He also warned the water in front about Yisrael, who do Bris Milah [on the Ever in front].