A reader of www.mimecanicapopular.com designed a solar water heater, low cost and easy construction allowed having free hot water and using black plastic bags.
This is how he built it: Insert a trash bag inside another and staple the ends open to a wooden dowel 1 “(2.54 cm) diameter by 36″ (91.44 cm) long. Wrap the bag around the post twice and hang from the ceiling. Make a hole in the top of the rafts, insert a garden hose and connect hose as shown.
The outside faucet filled the water bags and closes once the bags are full. Although the reader extended the hose to your shower through a vent in the ceiling and a dividing wall, the idea is appropriate for an outdoor shower, remember that even low water seal, so it will take some suction to start water flow.
Pumping is not required. The bags are filled with a garden hose connected to a faucet on the outside and the water flows by thermosiphon.