> you can toss it back into the ocean (with appropriate dilution)

... what are you going use to dilute the brine? The water you just desalinated? What's the point then?

Seems to me if your input is seawater, and your output is a separated-out set of {fresh water, saltier sea water}, and you put the saltier sea water back in the sea, you're going to create a localized concentration of high salinity, no? Seems kind of unavoidable to me.