Now that much of the world is running short of fresh water, desalination technology is greatly in demand.
Salt concentration in the ocean varies from about 3.2 to 3.8% so the job can be made 16% easier just by using water from the right spot. This unfortunately turns out to be at a depth of about 500m.
So today’s invention is to use a militarily-redundant nuclear submarine to desalinate water whilst moored at this depth.
Such machines already make their own fresh water but I imagine fitting out the interior with eg large-scale osmotic filters and flash evaporators, in place of the usual weaponry and crew accommodation.
The engines, which have already been heavily invested in, could provide many more years of useful service running the plant and pumping fresh water ashore.