I suddenly have the idea of using the earth's internal heat to purify water.
You will need:
* Supply of lava from fissure or volcano
* Heatproof ceramic pipe
* Steam Condensing Unit
Pump the seawater into the volcano, producing steam. Capture the steam. Let it cool in various radiators. Pump the now fresh and warm water to the consumers.
The salt in the water probably melts with the lava. Then what?