I have spend few years in San Diego doing hikes inland. There are decently sized mountains in the desert between say Borrego Springs and the coast. Not an optimal area to build a pipeline cheaply.

Pipe going along Colorado river should have less bumpy course but there is a biosfere reserve just on the top of Baja California. Not a good spot to suck giant volumes of sea water, even less so to construct a pipeline or heavens forbid dump the concentrated brine.