It is certainly easier to start with Blue Stocking's suggestions, as mine require a haul out
or a swim.
If all above seems fine then a culprit could be engine
alignment or your prop.
After that I guess you are left with the rudder
attachment to the hull
If you're going to dive & check the prop, though it works just the same hauled out, take something like a 2x4 to lay (or get fancy & clamp) against the hull
to within a 1/4 inch of the prop, while you spin the prop (by hand-DO NOT RUN THE ENGINE
). You can see some pretty insignificant bends with this simple device.