Is it permissible to benefit from the money of a father who receives a retirement pension from his work in customs, and is it permissible to perform ablution with water for which the bill has been paid?

The basic ruling concerning customs duties is prohibition, unless they are in exchange for services or due to the state's limited resources. However, their collection in most countries is an unlawful seizure of people's money, and it is considered among the forbidden mukoos (unlawful taxes) in Islamic law.

If your father had no financial source other than his work in customs, and it is highly probable that his salaries were not in exchange for legitimate services, then it is not permissible for you to benefit from his money.

If your father's money is mixed between lawful and unlawful, and the unlawful portion is predominant, then there are differing opinions regarding the permissibility of dealing with it, with some deeming it permissible and others forbidden. His money is closer to matters of doubt (shubuhat), and the Prophet, peace and blessings be upon him, commanded us to avoid them, saying: "Indeed, the lawful is clear, and the unlawful is clear, and between them are doubtful matters which many people do not know. So whoever guards against doubtful matters, he has indeed kept his religion and honor blameless. And whoever falls into doubtful matters, he falls into the unlawful…" And his saying: "Leave what makes you doubt for what does not make you doubt."

What has been said about the father's money applies to water bills paid by him. The mentioned prohibition applies in situations of choice. However, if you are forced to use this money and cannot do without it, then it is permissible to benefit from it to the extent of necessity.