The basic principle is that the water in swimming pools is pure unless it is known to be impure
Question

Is the water of a swimming pool pure if people’s clothes get wet, because small children swim in the water of the pool and they may make it impure (najis)?
Praise be to Allah.The basic principle regarding all types of water is that they are pure (tahir), unless some impurity falls into it and that impurity has an impact on the water’s colour, taste or smell, in which case we deem the water to be impure. Allah, may He be exalted, says (interpretation of the meaning):
{and We send down from the sky pure water} [al-Furqan 25:48].
An-Nasa’i (326) narrated that the Prophet (blessings and peace of Allah be upon him) said: “Water is pure and nothing makes it impure.” Classed as sahih by al-Albani in Sahih al-Jami‘, no. 1925.
The scholars of the Permanent Committee for Ifta’ said:
The basic principle regarding water is that it is pure, but if its colour, taste or smell is changed by some impurity then it is impure, whether it is a small or large amount. But if the impurity does not change it, then it remains pure. End quote.
Shaykh ‘Abd al-‘Aziz ibn Baz, Shaykh ‘Abdullah ibn Ghadyan, Shaykh ‘Abdullah ibn Qa‘ud.
Fatawa al-Lajnah al-Da’imah (5/84).
Based on that, if the water in the swimming pool remains as it originally was, namely pure, then it is pure unless we become certain that it has been changed by some impurity that fell into it.
Ash-Shirazi (may Allah have mercy on him) said in al-Madhhab (1/221): “If you are certain that the water is pure and are uncertain as to whether some impurity got into it, you may do wudu’ with it, because the basic principle is that it remains pure. But if you are certain that it is impure and you are uncertain as to whether it is pure, then do not do wudu’ with it, because the basic principle is that it remains impure. If you are not certain as to whether it is pure or impure, then you may do wudu’ with it, because the basic principle is that it is pure. End quote.
And Allah knows best.