«لَا يَبُولَنَّ أَحَدُكُمْ فِي الْمَاءِ الدَّائِمِ، وَلَا يَغْتَسِلْ فِيهِ مِنَ الْجَنَابَةِ».
Meaning: None of you should urinate in stagnant water, nor bathe in it for major ritual impurity.
The expression “stagnant water” refers to water that does not flow. The cited wording supports protecting such water from contamination.
The separate rule of a very extensive pond later in the passage hinges on size and separation, not on denying the general prohibition.