Regarding improvised solar heated showers, as noted above the water can easily become too hot.
Might it be worth the use of a thermostatic mixing valve ? These are widely available from plumbing supply houses.

They only work if you have a supply of hot and cold water, both under pressure.
But in the example given of a length of black hose in the sun, and fed from a pressurised supply, a second cold supply and a thermostatic valve should give good results.

Another option is a 12 volt submersible pump and a drum of warm water. This has the merit that the temperature of the water in the drum wont change significantly during the shower, and can of course be adjusted before hand.