X2 on the GI showers,
but, we have also used our outdoor shower and a small plastic pool. We all shower, and catch the water in the pool. We will use an old hose to siphon the water out and direct it toward a ditch or drain or whatever we can find close by.
I had a couple of old large hula hoops that we put shower curtain hooks on, had 2 old stained shower curtains (so there was plenty of fabric to go around) and hung it up in front of our outside shower. It worked great at the beach for "de-sanding" bathing suits without putting on a show for the neighbors.