Is Swimming in Mixed-Gender Pools Allowed in Islam?

It is not permissible for a man to use a swimming pool where women are also using the same pool, because it involves exposure of ‘awrah and unavoidable looking, which leads to fitnah.

References:

Fatawa Hindiyya (Vol. 5, p. 328):

يُكْرَهُ الِاجْتِمَاعُ فِي الْحَمَّامِ لِكَشْفِ الْعَوْرَةِ وَالنَّظَرِ إِلَيْهَا

Gathering together is impermissible due to exposure of ‘awrah and looking at it.

Radd al-Muḥtar (Ibn ‘Abidin) (Vol. 6, p. 371):

مَا أَدَّى إِلَى الْحَرَامِ فَهُوَ حَرَامٌ

Whatever leads to the unlawful is itself unlawful.

ONLY ALLAH KNOWS BEST