stator....
Check your stator vanes for damage or missing vane blades. When your out and at mid throttle, does the jet stream seem to have a circular motion.
The stator is designed to take away the vortex created by the impeller, to counter the spinning motion so that you have even response. Without the stator, the counter clockwise rotation of the pump will send the ski off to the right really bad.............so, look into the back of the nozzle to view your stator. It's a brass colored part, stationary set of vanes.