Add sea beans to the list -- they're delicious!

I'm not sure if your idea works overall though, rather than trading one problem for another. I'm sure there are places it could be helpfully done, but where would we find the physical space to grow enough saltwater crops to replace a large proportion of freshwater-driven industrial agriculture? Coastal and especially estuary habitat is fragile and critical to begin with, and the last thing we want to do is convert any of it from biodiverse native vegetation to a monoculture of human food, even if it helps slightly with the water problem.

Maybe this would be a good application for futuristic vertical farming tech in high-rises near the sea. I don't know which crops might be amenable to that.