Use a closed passive solar hot water system. No electric or pump needed. But the heating coils need to be lower than the water you want to heat. Ideally this would work if you have a south facing dam. Put the heating coils at the bottom of the dam. As the fluid heats it rises to the coils in the water near the top of the dam. The heat will transfer to the water and the cooled off fluid flows back down to the heating coils. If you can�t get the heating coils lower than the water level a solar circulating pump will also work.
I can envision a cheap DYI project using 1� black tubing, black painted plywood and some glass or Plexiglas.
