You may want to take a look at Cabela's catalog. There is a submersible pump that runs off of 4 D batteries, and has a little shower wand. It's sold under the "outback brand name" Basically, heat up some water with whatever source you choose and you've got an instant shower. Pumps out about a gallon a minute, and has a little switch on it + clamp, so you can hang it to turn it on and off. The whole thing, including the power pack is water proof. The four D. batteries last a very long time. I carry this little device with me mostly for transferring water from a stream or lake into a 5 gallon jug, but it is actually designed as a portable shower. Under 20  bucks from what I recall... 

If someone would just come up with something like a Coleman hot water on demand at half the size I would be a happy camper. It's the only hot water device that actually heats up 40� water to shower temperatures + turns the pump and burner on at once unkike the Zodi, which are very poorly made to me. 

Troy