Heating water with sunlight is relatively easy, mostly because of a physical phenomenon know as a Thermosiphon.
It describes the natural convection currents created when the liquid heats up and rises away from the colder liquid, moving the water around like a heat pump.
Because of this effect, solar heat absorbers can be very simple, with few or no moving parts. Gravity separates the hot from the cold water, and the hottest part of your reservoir is readily available for use at any time.
There are many possible designs for a wide range of budgets, with many DIY and commercial solutions available.
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Excellent 2.5m square thermosiphon water heater, using recycled fluorescent tubes for double glazing @ Youtube
Heavy-duty DIY solar heater slide-show with an amazing soundtrack and testing @ Youtube
Simple water tank style solar water heater DIY @ Youtube