In the third aliyah G-d says to Moshe: stretch out your hand over the sea and divide it, and the Jews will go forward in the midst of the sea over dry land.
Moshe stretches out his hands and the sea divides. (Actually, the sea divides into twelve paths -- one for each tribe. We are told that when Moshiach comes, may it be speedily in our days, the sea will similarly divide, but into only seven paths). Rashi says that the waters of the whole world divided along with the Yam Suf.
G-d removes Himself from leading the way before the Jews to following after them. This is in order to go between the Jews and the Egyptians to protect the Jews by absorbing the Egyptian's arrows and catapulted stones (says Rashi).
The pillar of cloud also goes behind the Jews to cause darkness for the Egyptians.
The waters are a wall on each side of the Jews and they walk on dry land in the midst of the sea.
The Egyptians pursue after them, including all of Pharaoh's horses and chariots.
G-d takes off the wheels from the Egyptian's chariots, and the Egyptians say, "Let us flee from Israel, because G-d fights for them".
