"In Arizona water prices are going up. "

As economics says they should. When demand outstrips supply, prices will rise.

"Sure, as an engineering problem, we can design pipelines to bring the ocean here. As a practical matter, it's economically unfeasible."

Rising prices will mean it will no longer remain economically unfeasible at some point. When prices rise, it opens up new avenues to approach problems that were not feasible before. Furthermore, it provides economic incentive for smart people to start looking at ways of doing things better or more efficiently.

"Technology can only go do far to save us from our own folly. After that mother nature will crack the whip."

Right, man will never be able to fly, mother nature will crack her whip. Humans simply cannot fly.

It is impossible for people in Australia, India, England, and the U.S. to be able to meet and converse on a project because a person cannot be in Australia and the U.S. at the same time. Mother Nature cracked her whip about people being in two places at once.

It used to be impossible to ship fresh foods around the country. Mother Nature had cracked her whip and food would spoil.

And on and on.

What is strange about your Mather Nature argument is that the water debate isn't even really about technology. We already have crude technology that allows humankind to use ocean water for daily consumption and use. It just isn't quite economically feasible yet (though it is close, especially for areas with an ocean nearby). It wouldn't take the price of water to rise much to make it definitively economically feasible. Plus, right now the technology involved is very crude simply because it has never really been a problem before. So now that it is, there are lots promising advances right around the corner.

Don't get me wrong. Water prices are going to rise. They have to. However rising water prices do not need to mean a shrinking population.