Is it permissible to work cleaning a swimming pool at night when no one is present, knowing that it is a mixed-gender place during the day where sins are committed?

It is not permissible to work in cleaning or maintaining the swimming pool if it is used in the manner described in the question, because doing so constitutes aiding in sin and transgression. Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." A Muslim must fear Allah, for the paths to lawful earnings are many. Allah Almighty says: "And whoever fears Allah – He will make for him a way out And will provide for him from where he does not expect."