I have got good results by pouring water into a dark garbage bag, enough for a military shower but so it lays flat and only 2" deep. laid out onto flat ground, leaves underneath if the ground is cold, around noon the bag collects a considerable amount of heat.

Once warmed sufficiently you can snip a corner and hang the bag, actually hang first then snip if you go this route. Or you can pour the water into a bucket with holes.

If your doing it a lot it pays to set up 2by4s on edge into a square and rig, glass, clear fiberglass corrugation, or plastic over top. The key hear is to surround the bag and place the clear cover in such a way that it maintains an inch and some of airspace in such a way that the air space is pretty well sealed.

We connected four lengths of 2by4 with hinges and used Plexiglas that we drilled on the corners so lag bolts and wingnuts held the cover on tightly. We sized the frame to accept a standard twenty gallon garbage bag and placed the frame and bag on top of a piece of iso-cyanurate insulation to keep the warming water off the colder ground.

As I remember it a batch went from about 45F to something around 130F, a warm (not quite hot) shower, in twenty minutes at noon. With two setups being run we showered eight people in about two hours. A few two at a time. Given none had had a shower in two weeks it was rave reviews all round. YMMV.

thats the staple washing method for "urban survival". if you have to save water just wet a towel with hot water and wipe down your entire body starting with your face and ending the the pooper.

look at msr dromedary water bags. they are industrial tuff, black (solar), and can double as a bullet proof water supply. shower adapters come separat, but they are simple (and simple seems to last a lot longer). there are other adapters too...gravity filtration, spigots, etc. it even won the "gear of the year" award in backpacker magazine!

i have the 10 liter bag, and hang it up at base camp. they come in several sizes.

msr dromedary bags

here is the shower adapter...

dromedary shower adapter

Thanks for the suggestions. I'm avoiding approaches that use mains power, batteries or gas cannisters. Susan's garden sprayer approach sounded interesting so I thought I'd try it.

It basically worked. It would work better if it had a spray attachment designed for the job. Its capacity was only 5 litres, which is small compared to the 20l that the black bags offer, but I found it adequate at the low flow rates I was using. My kettle is 1.5l, and I found that 1 kettle of cold water and 2 of boiling gave around 60C, which was adequate (in fact too hot if the water wasn't in a fine spray).

We found a plain old kitchen sink sprayer worked well. Just connect it to your water source and squeeze the trigger to spray so it stops when you let up, no turning valves on and off.