Let me start by posing a leading question that hints at my solution:
Do we really need to use freshwater everywhere in the household?
Surely we desire/need/expect freshwater from our tap for drinking and cooking, etc... What I propose is an expanded water infrastructure that allows a household to receive input water of two quality levels: (1) the conventional water we currently get from our tap, and (2) filtered saltwater to be used for other purposes.
What could we possibly use saltwater for in the household, provided pipes and fixtures could handle it?
- Watering GMO plants engineered to live off saltwater (bioengineers: start cracking the genetic code of seaweed).
- Flushing toilets. This one should be obvious.
- Dishwashers and Clothing Washers?
Perhaps sufficiently filtered saltwater could be used for washing dishes and clothing?
It should be noted that this is a pretty significant change to plumbing infrastructure and might initially be used when retrofitting a population-dense residential building in a city willing to try new crazy things (San Francisco?) or when building net-new residential buildings near a saltwater source.