Making clean water from seawater
That seems like a sound theory, if possibly impractical.
I would contribute the opinion that this, along with nation-supplying levels of desalination, would be quite feasible if the US was allowed to build many new technology breeder-type nuclear power generating stations. That technology has enough safety engineering included to be considered basically safe as coal power, with a much higher power density and output, zero emissions, and the giant added bonus that they are fueled by the spent nuclear fuel/waste we are currently storing from current older plant designs. It is essentially a century or more of free fuel on hand that we already have no clue how to permanently dispose of. Massively abundant and cheap electricity without the emissions cost of other generating methods makes things like seawater desalination that could provide clean freshwater to every human on earth at both high quality and very low cost possible. I really hope we get there someday.
For a detailed look at this option from multiple positions, this is a good article: