I have acquired an old water heater that was leaking from the valve.

No worries, no use for it, I was going to scrap it but got to reading about solar water heaters.

It looks pretty neat, build a 'greenhouse' and run the main water line through the tank inside the greenhouse.

Has anyone done it? Pros/cons?

you have to build a collector to heat the water first, recirculate it to keep it hot till needed.

worked at a solar mfr back in 76, pools and spas mainly.

can be done but is more complicated than just running some pipe to a tank.

We may be thinking about different setups.

What I was looking at was essentially a glass box with the bare tank inside and in direct sunlight. Run the water line into it for 'extra' storage and to warm the water up to the 100 degree range.

Not a true water heater, but a way to get more water from the ground and stored in case power goes out. We have a private well and the extra peace of mind an extra 50 gallons brings is nice.

I don't like worrying about firing up the generator and hooking up the well in a rainstorm if the power goes out on the bottom end of the pressure tank before the pump kicks on. I think it kicks on when it gets down to 15 gallons.

that will work but you need to put a relief valve on the tank (that box will get super hot in july)

the first cabin I lived in in colorado I had 2" black irrigation tubing laid in a large circle (300' or so) on the ground, that was my hot water, even in winter it would get hot by end of day if the sun was out.
the person that built the cabin set it up and was the only hot water except for the wood stove.

read up on solar hot water before you do it and you should be fine.

How will you keep it from freezing into a huge block of ice overnight with these 15degree temps ??

Trickle the water in the sink and shower

Not good if you have a septic tank.

Make sure you don't have hard water. I know one person that tried it and the pipes got limed up quick.

Paint it black, set it in the sun, HOT!

Built more than a few water filled panels with various rates of success.
Biggest problem is freezing in winter and little to no control of heat output sometimes resulting in very hot water and excess pressure in water lines in closed systems.

Solar PV hybrid system is way to go.
Solar PV panels don't freeze.

The best system uses 500-1000 watts of solar panels.
It uses a converter/controller that fits a standard 2 element 240V water heater and attaches to the standard bottom element and thermostat in an electric hot water heater.
Set lower thermostat 20 degrees warmer than the upper so it uses solar most of the time.
I generally set lower to 150 - WARNING: THIS IS HOT so be careful if you have kids in the house - and upper element that is still hooked to house wiring to 130.
If the sun is out, you get lots of hot water. If the sun isn't out, you still have hot water from upper element.
Adding an extra wrap of insulation to your water heater will help it hold heat longer.

Total system cost is right around 700 bux.
1000 watts of used panels will run you about 320 bux delivered (4 used 250 watt panels x $80 each delivered).
The controller runs about 300 more + some additional for panel mounts and simple wiring and a small box.
Most houses use about 500 bux worth of electricity a year heating water.
If you schedule your hot water use, you can heat your water entirely by solar.
The nice thing is the rapid ROI as you can pay for the system in first couple of years of use and have free hot water from then on.