Islamic scholars explain this fatwa in detail.

If a person is swimming in still water, like a well or a pond, and the water is small, then urinating in it is haram.

If the still water is large, then urinating in it is makrooh, meaning disliked, but not haram.

If a person is swimming in flowing water, like a river or the sea, and the water is large, then it is not haram to urinate in it. However, it is still better to avoid it.

If the flowing water is small, then urinating in it is makrooh because it makes the water dirty and impure.

This Islamic ruling is based on a hadith found in Sahih Muslim. It is reported by Abu Huraira that Prophet Muhammad said that no one should urinate in still water.

Because of this, Islamic scholars advise swimmers not to urinate in water.