This is maybe the most specific warning I've ever seen on a public sign, and it happens to be at the playground near the water features (or "sprayground") near my sister-in-law's house in Missoula.
You have to think that something very bad happened to prompt this highly descriptive warning. Like someone took the term "sprayground" too literally and unloaded on the daisy-shaped sprinklers and brightly-colored pipes, and it was so dramatic that it became seared in the collective memory of Missoula Parks & Rec.
Otherwise, why "diarrhea?" If you're an "individual" who is at the point where you're tempted to shit on a sprinkler, I feel like you shouldn't have to be told not to engage in water play.
I had a torts professor in law school who was fond of saying of warning signs and labels, "at a certain point, you can't learn everything you need to know about life from the back of a peanut butter jar."
The same holds true for this I think. At a certain point, you can't learn not to have explosive diarrhea all over a public water park from a sign telling you not to do that.