If you're not actually cleaning yourself--that is to say, if you're already clean, not if you're lazy--then there's no reason to be using any more clean water. There could be set up a switch that, when flipped, moved from a "use/dispose" model to a "use/recycle" (under the literal meaning of the term "re-cycle") one. It would, of course, build a small buffer at the beginning so it wouldn't just be spraying the last of your shampoo all over you, but that would be the last of the new usage for the day, and it could last as long as you want--only as dirty as a bath, but in motion!

why? the faster we waste resources the more motivated we'll be to come up with new solutions. necessity is the mother of invention.