Beshalach - The Power of Torah

So Moses stretched out his hand over the sea, and toward morning the sea returned to its strength, as the Egyptians were fleeing toward it, and Hashem stirred the Egyptians into the sea.
יֵּט משֶׁה אֶת יָדוֹ עַל הַיָּם וַיָּשָׁב הַיָּם לִפְנוֹת בֹּקֶר לְאֵיתָנוֹ וּמִצְרַיִם נָסִים לִקְרָאתוֹ וַיְנַעֵר ה` אֶת מִצְרַיִם בְּתוֹךְ הַיָּם
Shmot 14:27

What does this mean "the sea returned to its strength" (לאיתנו), couldn't the verse have just said the sea returned, or went back?

Says the Or HaChaim Hakadosh, this expression "the sea returned to its strength" (לאיתנו), means that the sea went back to its initial condition. Bringing the Midrash Rabba on Breisheet, we learn:
R' Yochanan says: Hashem stipulated with the Sea that it should split before Yisrael; thus it is written (Shemos 14:27), 'And the sea returned... to its strength (l'eitano),' to the stipulation (l'tano) that [Hashem made] with it.
Breisheet Rabba 5:5
The  Or HaChaim Hakadosh says that he has a problem with the reference to this condition here, rather the condition belongs to the time of the separation of the earth and waters. So perhaps the verse is coming to explain that after the water returned, the sea realized that it was created with a special condition in place. 

What Makes Water Split?
What was the nature of this stipulation that Hashem made with the sea? What were the conditions required for the sea to respond accordingly?
First, the Or HaChaim HaKadosh looks at how the sea responded when Moshe approached for it to split.
Moses went to divide the sea as G-d had commanded, but the sea refused to comply, exclaiming: "What, before you shall I divide? Am I not greater than you? For I was created in the third day and you on the sixth." 
Shmot Rabbah 21:6
The Or HaChaim HaKadosh then contrasts Moshe's confrontation with the sea, to the encounter of R. Pinchas Ben Yair with the river.
Once, R. Pinchas b. Yair was on his way to redeem captives, and came to the river Ginnai. ‘O Ginnai’ , said he, ‘divide thy waters for me, that I may pass through thee’. It replied. ‘Thou art about to do the will of thy Maker; I, too, am doing the will of my Maker. Thou mayest or mayest not accomplish thy purpose; I am sure of accomplishing mine’. He said: ‘If thou wilt not divide thyself, I will decree that no waters ever pass through thee’. It, thereupon, divided itself for him.
Chulin 7a (Soncino Translation)
 Why did Moshe seem to face a much more stubborn body of water then R. Pinchas Ben Yair?

Created with Conditions
The Or HaChaim HaKadosh explains that within the outset of creation itself were special conditions set within all of creation.
R. Jeremiah b. Eleazar said: Not with the sea alone did God make a stipulation, but with everything which was created in the six days of creation, as it is written, "I made the earth, and I created man upon it; as for Me-My hands stretched out the heavens, and I ordained their host." (Isa. 45:12).
Breisheet Rabba 5:5
In fact the creations were made to be subordinate to Torah, and those that endeavor in it. This means that the creations are subjugated to the will of the Torah and those learned in Torah, as if Hashem Himself were ruling over them. As it says in Vayikra Rabba, "Hakodesh Baruch Hu said to the world 'Who created you? Who created Yisrael? It was all out of the power of the Torah.'

Before and After Torah
Here The Or HaChaim HaKadosh explains the fundamental issue at work. For at the time of the departure from Egypt, it was still BEFORE receiving the Torah. So the initial conditions set with the creations at the time of creation were not active yet. This explains the reason for the protests of the sea against Moshe's request to split. The sea's claim that "I was created in the third day and you on the sixth" indicates that Moshe was not yet considered a Ben Torah - a Master of Torah.

Moshe split the sea not from his status of a Torah Master, but with the help of Hashem. As the midrash says, "From my hand I (i.e Hashem) stretched the heavens, and all their legions I have commanded,': I commanded the sea to split before Yisrael; I commanded the heavens and the earth to be silent before Moshe. (Breisheet Rabba 5:5)"  This gave Moshe the appearance of being a Ben Torah to the sea, as it says "from His right hand was a fiery Law for them" (Devarim 33:2), causing the conditions from creation to activate and the sea to split.

Yet after receiving the Torah, every Torah Master, every Tzadik carries with him a divine contract giving him power over creation. Thus we see the innate power and confidence that R. Pinchas ben Yair showed when he confronted the river, that as a true Torah Master he could demand that the river succumb to his will.