After trying a electric pump shower (Big Kahuna). hand pump shower (the late Zodi Extreme), I settled on this collapsable Nemo unit:
Helio™ Pressure Shower
Due to it's black color, the water will get warm enough on a 75ºF day if it's left out in the sun. And it can always be filled with warm water from the stove, etc. You can fill it with cold water and strap it to the roof rack, or carry it collapsed and small.