If you're installing a permanent structure, you'll want to make sure that it has a drain—and not just in case it rains. You'll want to be able to clean out the shower periodically and make sure there are no leaks, which can cause mold or mildew growth. A drain also helps with drainage, which is important for keeping your outdoor shower clean. Without one, water will pool up on the surface and create a breeding ground for mildew or mold. Not only does this look bad—it also makes it more difficult for you to clean the area around your shower as well as wash yourself off after using it.