Water towers aren't even close to the scale needed for reasonable energy storage.

A typical water tower holds about 4 million liters of water and is about 50m tall.

The Racoon Mountain Pumped Storage reservoir holds 410 BILLION liters of water (so 410 thousand times what a water tower holds) and has a head of 320m.

So you'd need about 1.6 million water towers with energy extraction capability to be equivalent to just that one pumped storage plant, ignoring the fact that you'd have to do /something/ with that water to extract the energy at the right time, when typically it's just drained out as people need it for faucets/showers/whatever.

On top of this, you'd be dropping a lot of the pressure from the water tower, which is like 99% of the reason that water towers exist, to provide adequate water pressure.