Use bigger wire going to your house, farm or castle.
When the motor is turned-on, it will draw many, many amps of current until it is up to speed. A capacitor won't help much unless it is huge and expensive. A high-voltage, high-value capacitor wired in parallel with the motor will lower the power-factor of that big inductance, reducing its power-draw and your electricity-bill a bit.
The voltage is dropping along the weakest wire between the power generating station, and the point where the wires to the pump and your lights join.