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.
The bacteria can from the air and water itself will growing and multiplies very fast. at a wet warm environment.
So please don't drink the hot water from solar water heater.
(Except pre-heated pressurized solar water heater, dual heat exchange split system water from up heat exchanger)