My wife and I lived on a sailboat for many years and we had only 100 gallons of water storage capacity. We used rainwater and we had a small water-maker (reverse osmosis). Anyway, we had to conserve.
Although we had a stand-up shower stall on the boat and even a water heater, we took most of our showers on-deck with an agricultural sprayer. We used a 10-liter size (big enough for a reasonable amount of water, but light weight enough to be easily manageable). We kept it wrapped in dark cloth in the sun to warm it up. For shower time, pump it up and wet down. Soap up and then rinse. The spray wand on ag. sprayers is too long for comfortable showering, so we cut it shorter only about 6 or 8 inches seemed about right. Release the pressure when done, so it does not leak. An ag sprayer used like that would last a few years.
Now we have a regular household shower and on-demand hot water. We recycle all our gray water and we have a hot-water bypass so the cold water in the hot pipe goes back to the cistern before we turn on the shower. We still are conservative with water, but we use more than we used to - but it is all rainwater we collected ourselves, so we're ok with that. Anyway, we REALLY appreciate a good shower now. But if water gets scarce, we would go back to the ag sprayer in a pinch.
Now I shower daily, but sailing - when I was not working too hard and getting sweaty - I would only shower every two or three days. My wife would not go so long - always a daily shower. I suspect there is a gender difference, and of course personal variations. That is just our experience. Except LOTS of sailors use ag sprayers for showering even if they have regular shower facilities aboard. Naturally those ag sprayers are devoted to showering; not ever used for pesticides or etc.!
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