Rashi: I began all my deeds and finished them, like one who digs a pit and finds foreign water and drinks. It is called foreign water because until now, no one knew about it.

Radak: I came to the source of the water. I had so many troops that the water on the ground did not suffice. It is called foreign water because people did not know about it or see it.

Malbim (Yeshayah 37:25): Chizkiyah stopped up the springs, so the enemy will not have water. I do not need the springs - I revealed [other] water sources and springs and drank from them!

Sanhedrin 95b: The first soldiers swam across the Yarden river. (Due to what they drank and the water absorbed in their garments), the middle ones walked through the water. The river was totally dry when the last ones came. They had to find a new source of water to drink - "Karti v'Shasisi

Rashi: If I besieged a secure city, I brought many troops against it and dried its rivers, through what they and their animals drank, and trampling of their feet. It says v'Achariv (future) because I do so constantly.

Malbim (Yeshayah 37:25): Chizkiyah gathered water within the walls of Yerushalayim, to be able to endure a siege. I have enough troops to destroy [what you saved] ¹ !

Refer to 19:24:1:4.

Rashi: They are rivers that supply besieged cities.

Radak: This is Ye'orim like that of Egypt (the Nile), a giant river. This is an exaggeration.

Malbim (Yeshayah 37:25): It is the water that Chizkiyah gathered and saved within the walls of Yerushalayim.