Maybe urine is sterile .... if one is not harboring some, even low-grade, infection, but I'm not buying it as harmless, nevermind beneficial. If there is too much yeast present, one can give oneself a good case of athlete's foot, not to mention contaminating the damn tub.
These lunatics keep pushing stuff the First World rightly grew out of the last century when toilets and showers were found to have their own particular and separate uses.
Remember "cleanliness is next to Godliness"? Yeah, that was to help keep bacteria and virusii at bay.