Solar hot water heater from GreenPowerScience.com with copper tubing. I understand that these kind of heaters can get the water up to about 100 degrees. Kind of limited there because it turns into steam at that point, unless you pressurize it, in which case it'll go up 180 degrees C. How hot did you say you wanted your shower? So realistically you want 20 gallons of solar heated water plus 40 gallons to cool it down to a usable temperature if you're doing it direct.

Solar water heater and the heat transfer material you want to use. All metals leach trace amounts of lead, zinc and iron, even copper. There are several reasons not to drink tap? water or shower water regardless of the heat source or plumbing source. The only truly safe water for drinking is steam distilled and reverse osmosis (safe, mineral OVER leaching, MYTH, not true, unless you are dehydrated or facing famine, extreme fast developed world diet, not possible). Truly pure water is hard to find. Copper is a good choice but expensive.