The problem is not just potable water, but also water for agriculture and to support wildlife habitat. When all the demands for water are taken into account, large scale engineering projects may be the best solution, moving water from where it is in excess (Canada) to where it is needed (California). Some Canadians may scream about it at first, but compensating benefits to Canada may make such a project politically palatable and genuinely beneficial to both countries.
I think Canadians would not be crazy about your idea. But wait, try it out on American residents of states bordering the Great Lakes. You will find little to no support for this idea among those people either.