What does "Gam At" include?
Rashi: Also you (Radak - Keneses Yisrael) will be saved with him (Mashi'ach), just like he is "Tzadik v'Nosha" (9).
Malbim: It includes Bas Tziyon and Bas Yerushalayim mentioned above (9). They are the Klal of the nation.
Why does it say "b'Dam Brisech Shilachti Asirayich"?
Rashi: In the merit of Dam ha'Bris that was thrown for you at Sinai - "Hine Dam ha'Bris Asher Karas Hashem Imachem" (Shemos 24:8), I sent you from Galus.
Radak: This refers to the blood of Bris Milah. Yisrael persisted in it in Galus more than all other Mitzvos.
What is the pit without water?
Radak: It is Galus.
Radak citing his father: Water is Nevu'ah. This is a Mashal that Nevu'ah was sealed in Galus. There was not another Navi after Chagai, Zecharyah and Malachi. It says about Nevi'im "Hoy Kol Tzamei Lechu la'Mayim 1 " (Yeshayah 55:1).
Malbim #1: There is no water in the Dam Bris - therefore I sent you out of the pit (Galus). During the Galus you were Moser Nefesh for Kidush Hashem, and refused to switch water in place of blood. [Nochrim] told you that it suffices to immerse or have water sprinkled on you in place of Dam Milah. You persisted in Torah creed.
Malbim #2: Via Dam Milah, I sent you from a pit without water. In Galus Bavel they were in a pit full of water - Bavel is a depth full of water - now I send you from a dry pit, i.e. Galus Romi.
Radak and the other Meforshim there explain that water refers to Torah. (PF)