Wouldn't it be possible to somehow recover the latent heat of the boiling water when it passes back into the liquid phase, minus the salt? I'm pretty sure I learned the theoretical maximum efficiency for this in my undergrad physics class but I can't quite remember.
Maybe you could use the steam to pre-heat the water that enters the system? If the water is at 60C instead of 20C to begin with, that will reduce the amount of energy needed to boil it. Besides, all that steam needs to be condensed back to water anyway if people are going to drink it. (I think that's what they already do in advanced "multi-stage" distillation plants.)
Theoretically, you could also add a steam turbine to make some extra electricity on the side, but I don't know how efficient such an add-on might be.
