Why does it mention "Mayim Tehorim"?
Rashi: I will atone for you, and remove your Tum'ah via sprinkling on you Mei Chatas, which removes Tum'as Mes.
Radak: This is a Mashal. Just like a Tamei person becomes Tahor via immersing in water and sprinkling on him Mei Chatas, so one who is Tamei via Aveiros is purified via Kaparah.
Radak citing Targum Yonasan: I will abandon [punishing for] your liability, like one who become Tahor via water that is thrown and the ashes of the Chatas cow. You will be purified from all Tum'ah, and from all your error (idolatry) I will purify you.
Malbim: This purifies from past sins. Nowadays, this is Tum'as Nidah ? "k'Tum'as Nidah Haysah Darkam Lefanai" (17); she needs to immerse in a Mikveh. Tum'as Mes required sprinkling with Tamei water (Mei Chatas is Metamei one who sprinkles or carries it 1 ). Sprinkling Tahor water would not help. However, then it will help. I.e. nowadays, what becomes Tahor in a Mikveh is via immersing the entire body in it. However, Tum'ah that departs via clinging to [another] Tum'ah, i.e. sprinkling water with;] ashes of Parah Adumah, requires Tamei water. In the future they will become Tahor via sprinkling a small amount of Tahor water 2 .
Malbim (Bamidbar 19:21): I.e. if not needed [to be Metaher a person or Kli].
Malbim (according to the Nimshal): With a small arousal and 'water of Da'as and Teshuvah, you will become Tahor. Nowadays, this helps only for Tum'ah of the body, but not for Tum'ah of the Nefesh, and only for Tum'ah, but not for acts of idolatry. In the future, I will purify you from all Tum'os - of the body, of the Nefesh, and from all your idolatry.
