Do not alternate the use of one collector between a pool and domestic water heating for health reasons. Plain old water is one of the best solar storage and transfer media available. Swimming pool solar collectors are easiest to make because they circulate pool water directly through the collector. Since the pool water is usually cooler than the air, heavy insulation and a glass cover are seldom needed. In contrast, when heating domestic water, the water temperature must be raised much higher than the air temperature.
Insulation is needed on the back of the collector with single or double pane glazing on the top.
If you can repair a bathroom faucet, you should be able to build your own solar collector. Special electric water heater tanks, with built-in solar heat transfer coils, are available from major manufacturers like Rheem. A thermosiphoning solar system design does not require a pump to circulate the water. A simple active water-type collector consists of a shallow insulated box with a clear cover. When it is completed, locate it in a sunny location on the ground instead of on the roof.