What is "li'Gzarim"?
Rashi (from Mechilta Beshalach 14:16): There were 12 channels for 12 Shevatim.
Radak: First He split the sea widthwise, half its width or more, and then lengthwise. Yisrael left on the same side that they entered. They entered from Midbar Eisam, and from the sea they went to Midbar Eisam (Bamidbar 33:6-8 1 ).
It says that they camped in Eisam, at the end of the Midbar. They passed through the sea, ha'Midbarah, and went three days in Midbar Eisam. (PF)
What was the Chesed of splitting the sea?
Radak #1: It was Chesed that there was one channel for each Shevet, lest they enter mixed up.
Radak: Since they came back to where they entered, they entered the sea only so the Egyptians would [enter, and] drown in the sea 1 , and to show His great Chesed, that Yisrael passed on the dry [floor of the sea], and the Egyptians drowned in it.
Malbim: Hashem could have saved Yisrael in other ways. Also, He hardened Egypt's hearts, so they would enter and drown in it - "Ki va'Davar Asher Zadu Aleihem" (Shemos 18:11).
