PS bis:

1. toxins generated by bacteria are not necessary filtered out by the kidneys (the penis and the vagina are not clean though), therefore urine might be sterile in some cases, but still be chemically hazardous.

2. Unfortunately, many rivers, lakes, seas and others are bacteriologically and/or chemically unsafe for humans, not because it is naturally so, but rather because humans made them so. It is indeed sadder than peeing in a swimming pool.