Why does it mention turning a rock into water?
Radak: Just like He has power to dry the wet - He made the sea dry land - He can convert a dry rock to be wet. He is the Master, creates everything and rules over all; His creations do not deviate from His commands. Also a mountain, which is the heaviest and most fixed of the creations, when He wants, it is light and moves quickly and dances like a ram. This is so all will know that He creates everything, and all is in His hand.
Malbim: I explained above (77:17) that when Hashem revealed Himself on the sea to split it, He appeared like the source of the element of water. Via this, the power and nature of water was Batel to its source. This revelation turned also a rock into water.
What is "Agam Mayim"?
Radak: It is a spring - "v'Hikisa va'Tzur v'Yatz'u Mimenu Mayim" (Shemos 17:6).
Malbim: It is a pool of water.
Is Chalamish the same as a rock?
Malbim: It is harder than rock. From a hard rock came a pool; from Chalamish, which is harder, came a spring that comes to (supplies water to) the pool.
Why does it say "l'Mayno Mayim"?
Rashi: The Vov at the end is extra, like "Chayso Ya'ar" (104:20).
