And the waters split: The splitting of the Sea was the culmination of the Exodus for several reasons:
• The Egyptian army was still intact after the Exodus, so the threat of re-enslavement still existed. At the sea, the people were being pursued by an army that naturally should have destroyed them. According to the Midrash, the Jews were outnumbered by the Egyptians 30 to 1. Only when the sea split and swallowed the Egyptians alive was the process of Exodus truly complete.
• G-d had promised Abraham that his descendants would leave their exile with great wealth. Although the Jews took a large amount of wealth out of Egypt, they technically only borrowed this wealth from the Egyptians. It did not actually belong to them until after the Egyptians drowned in the sea. Furthermore, the Egyptian wealth that the Jews took at the sea exceeded that which they took with them from Egypt.
• Spiritually, G-d's promise to Abraham was that the Jews would liberate all the sparks of holiness trapped in Egypt. Since it was primarily at the sea that the wealth of Egypt became theirs, it follows that the primary release of the sparks also occurred at the sea.
• Finally, the ultimate goal of Exodus, the receiving of the Torah, could only have been achieved by first experiencing the spiritual revelations at the splitting of the sea.
Spiritually, leaving Egypt means freeing the Divine soul from the shackles of the animal soul. The Divine soul is free when it realizes that although we are surrounded by "pharaohs," we need not be servants to them; we are free to be servants of G-d alone. But the soul cannot experience this freedom fully until "the sea has been split," until the true, Divine nature of reality has been revealed.
Even before we become aware of the true nature of reality, we can "leave Egypt," break out of our bondage to materialism, and dutifully fulfill our responsibilities. But such uninspired commitment is vulnerable. If our mind and heart are not in it, Pharaoh and his armies can give chase. We truly left Egypt and Pharaoh behind only after the sea split, when G-d's presence is revealed and obvious and the lie of Egypt has been exposed.
It is for this very reason that, just as we are required to mention the Exodus from Egypt in our daily prayers, we are also required to mention the Splitting of the Sea, since it is an integral part of our daily spiritual exodus.