Or HaChaim on Exodus 14:26/אור החיים על שמות י״ד:כ״ו

1 נטה את ידך..וישובו המים, "Incline your hand so that the waters may come back, etc." Why was it necessary for Moses to incline his hand in order to make the waters come back? Surely, the division of the waters was only for the time it took for the Israelites to cross? G'd had not revised the laws of nature permanently! In fact Shemot Rabbah 22 states that when the last of the Israelites ascended from the sea, the last of the Egyptians entered the sea, implying that the sea would come back by itself.

2Perhaps the sea reasoned that the area where the Jews had crossed was meant to remain a crossing and that just as the Israelites could cross so could the Egyptians. Actually, G'd had decreed that the waters should not close immediately behind the Israelites in order to lure the Egyptians into pursuit. This is something the sea was not aware of. The sea did not concern itself with this, having kept its original promise to G'd at the creation to split when the time came. When Moses inclined his hand this was a signal to the sea that it had completed its task.

3We have a rule in Masechet Yom Tov 5 that if a collegium of judges has made a ruling of even a temporary nature, this decree remains in force until a collegium of at least an equal number of judges declares the original ruling as void. This is also what happened here.

4Another reason why it was necessary for Moses to incline his hand was simply that though the waters of the sea would indeed come back on their own, this would occur at such a slow speed that the Egyptians would be able to escape. Indeed, we have an opinion that some of the Egyptians who were close to shore were able to save themselves. As a result of Moses inclining his hand the waters returned with a rush as indicated by the word לאיתנו. Alternatively, seeing Moses had been the agent who struck the sea it was only fitting that it should be he who restored the sea to its original position. We find support for this view in Isaiah 63,12.

5Still another reason why G'd asked Moses to incline his hand prior to the waters returning was to give the sea a signal not to allow a single rider or chariot of the Egyptians to escape. Not even those who were close to the shore should be given a chance to save themselves. We know that the waters complied as the Torah writes that "the waters covered them and that not a single Egyptian survived."