[JANALA]

Q: Why do Islamic healers use water ?

A: Using water in treating illness is permissible in Islam. In fact some mu'allij (a person who treats with Islamic medical sciences) use water as the main ingredient in treating patients (apart from ruqyah)

ALLAH swt says,

وَيُنَزِّلُ عَلَيْكُمْ مِنَ السَّمَاءِ مَاءً لِيُطَهِّرَكُمْ بِهِ وَيُذْهِبَ عَنْكُمْ رِجْزَ الشَّيْطَانِ وَلِيَرْبِطَ عَلَىٰ قُلُوبِكُمْ وَيُثَبِّتَ بِهِ الْأَقْدَامَ ...Al-Anfal, 11: (Remember) ... He sent down upon you from the sky, water (rain) by which to purify you and remove from you the evil of syaitan and to make steadfast your hearts and plant firmly thereby your feet.

the word "ليطهركم" (to purify/clean you) comes from the root word "طهارة" and "طهور" that our Nabi s.a.w. had used in one hadith, from Ibn 'Abbas r.a.huma

ان رسول الله صلى الله عليه وسلم دخل على رجلٍ يعوده فقال لا بأس طهور إن شاء اللهVerily Rasulullah s.a.w.visited a sahabi who was sick, and he (s.a.w.) said; Dont worry, it will be cured insya ALLAH
- hadith Bukhori, Kitab al-Marda, hadith #5662

This demonstrate clearly the usage of water (rain water specifically mentioned here) in curing/healing/treating problems related to syaitan and heal the soul (heart).
(Dato' Dr Haron Din, Islamic Medicinal Studies vol2., 2011)