Even though you are feeding a 230 volt motor with 115 volts, it is still at 60 cycles and wants to spin the motor at 3600 RPM. The low voltage doesn’t supply enough power to do that so it never gets to full speed. The difference between the RPM that the power is trying to spin the motor and the actual RPM is called “slip”. The more slip, the more the rotor bars are passing through lines of magnetic flux, and the more heat it produces. Add to that a motor on a jet pump is air cooled with a fan, and the fan is not spinning at optimum speed. A submersible motor is water-cooled and can take it for a longer period of time. But it is always advisable to keep the voltage within plus or minus 10% of the rated voltage for the motor.