Might be true in the US, but here in Europe we see this quite often. Prices don't have to be negative, it is enough if they are cheap (we pay for water too). And in some locations there are already projects like this [0] where they built a hydro-electric pump station with a dam for storage plus a desalination plant that fills the reservoir from seawater in one location.

[0]: https://www.ree.es/en/ecological-transition/storage