Passing urine is a natural process of eliminating waste from the body. Many euphemisms for urinating include: pee, piss, take a leak, use the johns, and plenty more.
Each of us has our favourite. And then there is the matter of where to pee. Most of us pee in a formally designated place, like the toilet. But you will find folk peeing almost anywhere, and everywhere. The shower isn't an exemption.