Feel free to pee into it, but for the love of any higher power of your choosing please just don’t relieve yourself while waist deep in the water. If you do this, you may receive a visit from the Candiru fish.
This little guy has the nasty habit of seeking out other fish by the presence of excreted ammonia and urea, then burrowing into the gills using sharp barbs and drinking off the fish’s blood. If you couldn’t piece this together already, urine contains exactly what attracts the Candiru and the urethra is an amazingly accessible space for it to occupy.
Ladies and gentlemen, you have been warned.