1. Pouring water over one’s head.

Proof:

One of the Companions of the Prophet sallahu alayhi wa salam said:

لقد رأيتُ النَّبيَّ – صلَّى اللَّهُ علَيهِ وسلَّمَ – بالعَرْجِ ، يصُبُّ علَى رأسِهِ الماءَ وهو صائمٌ منَ العطشِ ؛ أو مِنَ الحرِّ

“Verily I saw the Prophet sallahu alayhi wa slaam at ‘Araj pouring water over his head due to thirst or heat and he was fasting”

[Graded Saheeh by al-Albani in Takhreej al-Mishkaat no. 1953]

Imam al-‘Adheemabaadi rahimahullah said:

فيه دليل على أنه يجوز للصائم أن يكسر الحر بصب الماء على بعض بدنه أو كله ، وقد ذهب إلى ذلك الجمهور ولم يفرقوا بين الاغتسال الواجبة والمسنونة والمباحة

“In this Hadith is evidence for the permissibilty for a fasting person to break the heat by pouring water over some of his body or all of it. The majority of the Scholars take that view and they do not differentiate between an obligatory shower, recommended one, or permissible one”

[‘Awn al-Ma’bood]
2. Gargling Water in the mouth and spitting it out: 

Imam al-Bukhari rahimahullah mentions in his Saheeh:

وقال الحسن لا بأس بالمضمضة والتبرد للصائم

Hassan al-Basri rahimahullah said: “There is no harm in the fasting person rinsing the mouth and cooling off”

Proof:

وَكَانَ الْحَسَنُ يُمَضْمِضُ، وَهُوَ صَائِمٌ ثُمَّ يَمُجُّهُ، وَذَلِكَ فِي شِدَّةِ الْحَرِّ

Hassan al-Basri used to rinse his mouth and spit it out and he was fasting. That was due to the severe heat.

[Reported by Abdurrazaq in the Musannaf no. 7505 and the Chain is Saheeh]

So this is an action of the Salaf, but you must not swallow the water because that breaks the fast if it is done intentionally.
As the temperature continues to rise, we can use the above practices to help us cool off in this Blessed month.

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan