Why does it say "Ki Im Sham"?
Rashi: Ki Im refers to "Bal Yisa Yesodosav" and "Bal Yinaseku" (20). It will not be bad, only good.
Radak: Here, "Im" means Emes. The same applies to "Ki Im Yesh Acharis" (Mishlei 23:18), "Hafkechem Im k'Chomer ha'Yotzer Yechashev" (above, 29:16). There, Hashem will be our mighty One; He will overcome Machaneh Ashur, even though it is great and awesome.
Malbim: This explains why the tent will never move. Hashem will be our mighty One and protection there.
Will there be rivers in Yerushalayim?
Rashi: Yes - "va'Yimad Elef Nachal Asher Lo Uchal La'avor" (Yechezkel 47:5), "u'Mayan mi'Beis Hashem Yetzei" (Yo'el 4:18). It is the river that gets mightier.
Radak: Hashem [will protect us] as if there are wide rivers around Yerushalayim that the enemy cannot cross, even in boats with oars or great ships cannot go in them or cross them. Since Machaneh Ashur was compared to a great river - "Mei ha'Nahar ha'Atzumim veha'Rabim Es Melech Ashur" (8:7). Hashem will be for them the place of a greater river that Melech Ashur cannot cross to conquer Yerushalayim.
Malbim: It means that even if the place would be converted to a great river, and the tent would be like a ship tossed about in a turbulent sea around it, and Melech Ashur is like a great ship that goes to seize the tossed boat, Hashem will stop boats from going even in rivers where they normally go. All the more so they will not go in a serene pasture! The Nimshal is, also Yerushalayim, which was serene, the great river (Melech Ashur) passed through. The king was like a great ship amidst the sea of his camps, to sink the tiny boat of Malchus Yehudah. Hashem did not let him pass.
What are "Ani Shayit" and "Tzi Adir"?
Radak: They are boats with oars, and great ships, respectively.
