However, did come across the portable Nimbus Watermaker WM5-50 (left) for the surprising low cost of only $371, but each time your replace the filters will be another $140, and it's unclear how often that might be required.
EDIT - My mistake, the Nimbus above is for freshwater, so that's no help.
Anyway, time will come my Grampian 26 will be my live-aboard for a time, or longer. And though I don't foresee any real difficulty maintaining a fresh supply of water for drinking and cooking, as can just carry a gallon or two each time back from shore to keep reserves topped off. (I don't think I'll be crossing oceans anytime soon.)
Still, don't want to waste that supply on showers, or dishwashing, or flushing the toilet.. and I mostly do not care for resorting to showering in saltwater...
My research has thus been brief, but I already know it will be necessary to incorporate a simple rain catching method to easily maintain an ample supply of clean water on my boat. It's been done for countless centuries for free, and is still so today.
I'll be looking into this closer soon enough.