I lived for a year in a house (in the tropics, in Indonesia) that had a solar water heater setup. I never saw the entire guts of it, but I think the basics consisted of a barrel and a very simple solar collector of pipe in something like a cold frame. It supplied more than enough very hot shower water for us on all but the few completely rainy days...which, rather unfortunately, were exactly the days when a hot shower would be the most welcome.
A little googling should show a variety of contraptions along these lines. If you get the positioning and plumbing right, you can have the water circulate from the collector to the tank via convection, which makes it about as simple and maintenance-free as is possible. Something like a toilet float valve to fill the tank automatically would not be a bad idea.
If you use a metal tank and other appropriate materials, you can perhaps rig up a grate underneath it and build a wood or charcoal fire to heat water at times when solar energy fails you.
