When will you pass through water?
Rashi: When you passed through Yam Suf [I was with you].
Malbim: There are two kinds of salvation. (a) Natural salvation. Hashem leads and guards him, so the damager does not approach him. "I am with you", so you will pass through water or fire b'Shalom. (b) Miraculous salvation. Even though he is in a place of danger, the nature of water or fire will change, and they will not harm him. About this, it says "uva'Neharos Lo Yishtefucha", "v'Lehavah Lo Siv'ar Bach." The former pertains to "Bora'acha Yakov", and the latter to "Yotzercha Yisrael" (1).
What rivers did not flood you?
Rashi: You lived among the Egyptians and Bavliyim who are numerous like a river, and they could not eradicate you.
Radak #1: Sancheriv is compared to a river - "Es "Mei ha'Nahar ha'Atzumim veha'Rabim Es Melech Ashur" (8:7). "Shataf v'Avar Ad Tzavar Yagi'a" (8:8) refers to Anshei Yerushalayim. He will not flood them, and his fire will not burn them. He is compared also to fire - "va'Telahatehu mi'Saviv" (42:25).
Radak #2: If this refers to the future, when you leave Galus, you will pass through nations that are like rivers or fire; they will not harm you.
When will you go in fire?
Rashi: In the future, "Hinei ha'Yom Ba Bo'er ka'Tanur" (Mal'achi 3:19). I will make the sun beat on Resha'im, "v'Lihat Osam ha'Yom ha'Ba" (ibid). Also then, you will not be singed!
Radak: Refer to 43:2:2:2-3.
