i feel like there’s probably a way to make water push a piston.

Okay yes, but you can't carry an entire river around just to run your car off of.

There is a thing where solar power is used to run a pump and water is pumped uphill or in to a tank during the day. Then when the sun is off the water is allowed to flow downhill over a hydroelectric generator, allowing a system that provides steady power all day and night without relying on really complicated shit like batteries.