Rashi: I began all my deeds and finished them and succeeded, like one who digs a pit and finds foreign water and drinks. "Karti" is based on Mekor (source).

Radak (Melachim II, 19:24): 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: 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 city that trusts on its rivers [to be able to endure a siege], 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: Chizkiyah gathered water between the two walls of Yerushalayim, to be able to endure a siege. I have enough troops to destroy [the outer wall, and eradicate your water]!

Refer to 37:25:1:4.

Rashi: They are rivers of (that supply) cities that I besiege.

Radak (Melachim II, 19:24): This is Ye'orim like that of Egypt (the Nile), a giant river. This is an exaggeration.

Malbim: It is the water that Chizkiyah gathered and saved within the walls of Yerushalayim.