What is the ruling on praying on parks, knowing that they are watered with water that has an offensive odour?

So long as there is an offensive odour coming from it, then praying in them (the parks) is not correct, because one of the conditions of prayer being correct is that the place where the Muslim prays should be taahir (pure and clean). If he spreads out a thick and taahir (pure) mat, then his prayer will be valid. 

But it is not permissible for Muslims to pray in parks – even on thick and pure mats – rather they have to pray with their Muslim brothers in the houses of Allaah – the mosques – of which Allaah says (interpretation of the meaning): 

“In houses (mosques) which Allaah has ordered to be raised (to be cleaned, and to be honoured), in them His Name is remembered [i.e. Adhaan, Iqaamah, Salaah (prayers), invocations, recitation of the Qur’aan]. Therein glorify Him (Allaah) in the mornings and in the afternoons or the evenings,

Men whom neither trade nor sale (business) diverts from the remembrance of Allâh (with heart and tongue) nor from performing As-Salaah (Iqaamat-as-Salaah) nor from giving the Zakaah. They fear a Day when hearts and eyes will be overturned (out of the horror of the torment of the Day of Resurrection).

That Allaah may reward them according to the best of their deeds, and add even more for them out of His Grace. And Allaah provides without measure to whom He wills”

[al-Noor 24:36-38]

And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever hears the call (to prayer) and does not come, then his prayer does not count, unless he has a (valid) excuse.” (Narrated by Ibn Maajah, al-Daraaqutni, Ibn Hibbaan and al-Haakim. Its isnaad is (saheeh) according to the conditions of Muslim). 

A blind man asked the Prophet (peace and blessings of Allaah be upon him), “O Messenger of Allaah, I have no guide to bring me to the mosque. May I have a dispensation to pray in my house?” The Prophet (peace and blessings of Allaah be upon him) said to him, “Can you hear the call to prayer?” He said, “Yes.” He said, “Then respond.” (Narrated by Muslim in his Saheeh). 

And there are many ahaadeeth which say the same thing. 

What the Committee for Enjoining what is Good and Forbidding what is Evil should do is to disallow people to pray in parks, and tell them to pray in the mosques, in accordance with the words of Allaah (interpretation of the meaning): 

“Help you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety)”[al-Maa’idah 5:2]

 and 

“The believers, men and women, are Awliyaa’ (helpers, supporters, friends, protectors) of one another; they enjoin (on the people) Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do), and forbid (people) from Al-Munkar (i.e. polytheism and disbelief of all kinds, and all that Islam has forbidden)” [al-Tawbah 9:71]

And in accordance with the words of the Prophet (peace and blessings of Allaah be upon him): “Whoever among you sees an evil action, let him change it with his hand (by taking actions); if he cannot then with his tongue (by speaking out against it); and if he cannot then with his heart (by feeling that it is evil) – and that is the weakest of faith.” (Narrated by Muslim in his Saheeh).