What is the meaning of "Porarta"?
Radak: You crushed. This is like "Por Hisporerah Eretz" (Yeshayah 24:19).
Who are "Rashei Saninim"?
Rashi: They are Egypt, who are called Taninim - "ha'Tanim ha'Gadol" (Yechezkel 29:3). Radak - that verse discusses Pharaoh. Malbim - they believed that they have power via the water; they worshipped the Nile and the alligators in it - "Li Ye'ori va'Ani Asisini" (ibid).
Why does the verse mention previous miracles that Hashem did?
Radak: You used to do all these salvations for us.
Malbim: Splitting the sea and making it dry for Yisrael was "b'Kerev ha'Aretz" (verse 12), and not via nature and the Ma'arachah. It was a double miracle - at the same time that it was dry for Yisrael, You drowned the Egyptians in it!
