For one the "water" moves near light speed effectively and tanks andand reservoirs are far more expensive, and gravity doesn't meaningfully apply. And there is some leakage with distance. And you cannot use a turbine to generate water from torque. Even if many equation structures are the same as fluid mechanics the assumptions break from differences.
This is not how the hydraulic analogy works. The "speed" of the water is irrelevant. The analogy is used to understand losses, energy balance, energy conservation, reactive power, etc.
