From David In Israel: On Passive Solar Water Heating

James
After Passover the weather in Israel always turns hot and clear. Perfect weather to save money.
American hot water heating is always on and is always wasting electricity or gas.
Passive solar is cheap and simple plus it can be supplemented with electrical on cold days.
It is law that every home have a passive solar water heater so I get to see the design on every house,
being in the West Bank I also see that the Arab homes and mansions will always have at least one dud shemesh (sun water heater)
If a commercial model is too expensive the construction would be simple and cheap: The collector is about 1.5M x..75M x 7-to-8cm deep panel placed at the lowest south facing part of the roof. Hoses carry the water up to the "dud" or tank which is as high as possible on the roof Israeli tanks are insulated, cheaper Arab models are simple black plastic tanks on stilts. Convection carries colder water from the bottom of the tank into the collector and hot water rises to the upper intake on the tank. On cold winter days we set the timer to run the immersion heater built in to the tank as on days near freezing the heater barely produces luke-warm water.
A wood framed glass paned collector with Black painted PVC pipe would likely make a great collector and a plastic barrel would probably make a good tank with a little Bondo work, similar to the Arab heaters.
Insulating the tank would make for warm water in the early morning. If a tank were suspended above the chimney it might be able to grab a few more precious BTUs from the hot gasses during the winter, get creative and let us know. Israeli water heaters will run near boiling. It is possibly easier to heat and insulate a small amount of very hot water than large tanks of less hot water.

On a related note: Your body also a solar collector. Avoid heat related injury. Remember that as hot weather approaches to keep hydrated!! If your urine is not clear and colorless you do not have a sufficient internal water storage and you are storing up toxins that your body wants to eliminate. Make a pre-hydrated body part of your survival kit!
You should always have at least a liter of water in your hand in your pack or next to you even when you are inside.