Solar water heaters continue working several days, the water temperature keeping very high for long time; it will be boiling at the day time and getting cold at the night. When water temperature maintained at 60-70 degree, this temperature region is an excellent environment for microbe and bacteria growing and multiply. This will affect the water quality it's not good for bathing and drinking.
Even if the solar water heater interior material using food grade material, but this not guarantee that water is drinkable.
Solar water heater is not a closed system, it's opening to the air, and the bacteria can came from the water itself also. When the environment is good, it will growing and multiplies very fast.
(Except pre-heated pressurized solar water heater, dual heat exchange split system water from up heat exchanger)