While on a boat, the map marker seems to show the direction the player is facing with a fixed angle (to the right of the shown map). what looks like happening is that while on the boat, regardless of which direction the boat is actually facing, the marker shows one fixed direction. But looking around with the mouse while on the board makes the marker move relative to the fixed direction.
1- Equip a map with a marker (i.e. made with a compass) while on a boat
2- Observe the marker, it points to the right of the map
3- Turn the boat
Expected: the marker turns with the boat
Actual: marker is fixed to point to left
4- Use mouse to look around without turning the boat, look degrees to right, observe marker.
Actual: The marker points to the bottom of the map.
So the issue does seem to be the marker direction ignoring the direction of the boat.