This explains a simple method of recovering energy from a waterwheel using only a low cost energy saving soft starter ( https://www.inverterdrive.com/group/Soft-Starts/ ) and a simple three phase AC induction motor.
The motor must be coupled to a waterwheel with such a ratio that the normal speed of the motor shaft is just above its synchronous speed. The 'Soft Starter' is used to flux the motor during its normal starting setting and then by-passed as usual. The motor is then generating electricity into the main supply as long as the main supply is present.
There must be some form of load control for the motor to maintain its optimum generating level and prevent it from exceeding its full load capability. This would be by an external method of water flow control, either by river level or via a controllable sluice. This has to be the lowest cost method of Generating Electricity from a Hydraulic power source.