That's a horrifyingly bad idea for so many reasons. I've always found Wim Hof kinda quacky in general, but this is beyond the stupidity I would expect from even a serious woo practitioner.

Even without wrecking your intestines, the bacteria and probable pollutants in a public fountain would be disastrous for your intestinal flora. So, even without the risk of traumatic injury, it'd be really stupid to treat a public fountain as a mega-bidet. Not to mention you're putting others at risk. Kids and pets often play in fountains, even if they aren't supposed to.

I really can't even with this dude. And, lots of people, including nerds, take him seriously. It makes no sense.