Look at this outdoor shower by 'Viteo-Outdoors'

You attach a water source; (In my back yard it would be a hose), and when you stand on it, the water sprays upwards. I think this idea is an effective use of space and water management for the plumbing is minimal and the water on/off switch is gravity driven.

I found this 'gizmo' at notcot.org

Here is the designer's site: [Link]

And that must be Amanda Beard demonstrating how to use it.