Hi Andrew,
I have made some progress (I think)… Regarding the compass I did follow this article: https://ardupilot.org/copter/docs/common-compass-setup-advanced.html
At first I could not figure out why the external compass was automatically set to yaw270 although the arrow on the module was pointing straight ahead. After some searching on this forum I found this thread: Compass Orientation automatically set wrong
So I disassembled the external module and I could confirm that the compass is actually rotated 270 degrees off the white arrow.
After this finding I recalibrated the compass and accepted the yaw 270.
The current situation is that I can Loiter in a stable manner for maybe 15-20 secs, after that the orange line shifts 45 degrees (before it was 90) and the boat isn’t stable any more…
I’m now learning how to review tlogs but I could need some help in finding the right parameters to focus on.
Regards ///Jonas