Saltwater damages turbine blades very fast. Using saltwater would require some very different engineering for the turbine blades, which might not be worth the benefit of being able to use saltwater.

Turbines are precision machines. And if we’re talking utility scale installations here, we want an upper reservoir measured in thousands of cubic meters, not liters.

The pipe and turbine would break so fast it’s basically useless. Also, tanks are waaaay more expensive than digging a hole in the ground.

There’s good reasons why freshwater open reservoirs are used. If tanks and brine were usable, people already would be using them.