You could farm with desalinated water, but the economics don't support it so no one would bother (or at least the current farmers with their current crops would not). We're 'running out' of free/extremely-subsidized water supplied by the government for mostly-low-value crops. No one will ever grow alfalfa with desalinated water that then needs to be moved via canal/aquaduct - the beef from the cows you fed with it would cost like $2k/pound.