I think you should use the method I demonstrated. The screens above should demonstrate how it’s the same as your movie. That will protect you from errors from trying to find the Z position of the buoy before you know which XY position to check for Z…
However, your question about translation->rotation is answerable-- just trigonometry-- and worth knowing about anyways:
So if you want, you can have the buoy at a rest pose horizontal with its parent, and then determine the angle from the projection-- the distance along the Z axis. You could have it at some other rest pose, but then you’d need to do a bit more math (just algebra, subtract the angles found from two different projections.)
Of course, drivers work in radians, not degrees. There are 2*pi radians in 360 degrees.