why isn’t ocean water filtered and used when somewhere is having a water crisis?
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It is! Desalination is used extensively in truly water-scare places, like Israel, Saudi Arabia, Western Australia, etc. However, these systems do have some challenges, political, environmental, and practical.
First, the political. Desalination plants are large, expensive projects which take a long time to build, and a longer time to plan and approve. The conditions which are creating your water crisis may not be around by the time your project is built, eroding public support for them.
Second, the environmental. Desalination, even assuming you can get the energy from renewable sources, is not without environmental impact. You’re sucking up massive amounts of water, filtering some of it out as clean, drinkable water, and flushing the rest out back into the ocean at a higher salinity than you took in. At the inlet, you’re sucking up and crushing lots of different types of ocean live, and at the outlet, you’re dumping a flow of water which may be harmful to the species near to your shore.
Finally, the practical. Having a giant tank full of filtered water right at the shoreline may not actually suit the purposes of your water demand. For example, 75% of California’s water demand is driven by agriculture, which is (generally) far inland of any body of water which could conceivably be treated. How, exactly are you going to get it there? Operating pumps along huge pipelines adds yet another tremendous expense to your water project, concomitant with obtaining it.
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