I doubt that the duck tail would help with the autopilot hunting and
it will make the boat slower. If the pilot is hydraulic I would check
the pilot's rudder angle sensor to make sure it is operating correctly
and is properly centered. Fiddling with the gain controls may help
and the manual should have some set-up procedures to follow. A
gyroscopic rate change sensor can be added to many autopilots these
days. They can reduce autopilot induced oscillation and also help
with radar targeting.

-- Tom.