Many people live near the ocean. It doesn't have to solve every problem. Also I don't think many people are living hundreds of miles above sea level :)
The problem is it solves basically none of the issues.
Where people live isn’t where people consume aquifers faster than they are replenished. Across much of the east coast farmers don’t bother with irrigation because even the cost of pumping water from a well and delivering it to their fields isn’t worthwhile.
West of the Mississippi the amount of rainfall drops and therefore the need for water skyrockets. https://us-canad.com/rainfall-usa-map.html The difference between 150 inches per year of rainfall and less than 25 is huge.
Where people live isn’t where people consume aquifers faster than they are replenished. Across much of the east coast farmers don’t bother with irrigation because even the cost of pumping water from a well and delivering it to their fields isn’t worthwhile.
West of the Mississippi the amount of rainfall drops and therefore the need for water skyrockets. https://us-canad.com/rainfall-usa-map.html The difference between 150 inches per year of rainfall and less than 25 is huge.
At the same time the average altitude above sea level also increases. https://gisgeography.com/us-elevation-map/
So if you want to get water where it’s needed in the US. That’s mostly up hill and a long way from the sea.
PS: Coastal cities can always out bid farmers for water, or use some desalination but that’s a tiny fraction of the overall water usage.