I would not think the tunneling would be worse than NY, NY water supply. Deeper maybe but not much more of a project.
There and back would be 4 times the distance through more rock and deeper depth, plus a desalination plant and steam driven pumps along with massive lift stations for any deliveries along the way, and it would not be much more than a simple gravity drain?
I’ve worked projects that had similar thoughts for cost estimates.
They didn’t go so well.