For this system you could use a 'dummy' structure between the vessel and the spar. Connect the spar to the dummy structure using a ball and socket joint, so that the spar can rotate freely in all directions. Then connect the dummy structure to the vessel using a hinge joint, where the hinge axis is parallel to the XY plane and is positioned a long way away (e.g. 1000 m) from the vessel/spar. In this way the motion of the dummy structure relative to the vessel is almost linear in the Z direction as it travels along a small arc of a large circle around the hinge axis (assuming that there is not a very large relative Z motion between the vessel and spar).
To create the dummy structure you can add an extra part in the geometry editor (it doesn't matter what the part looks like), import the updated geometry into Aqwa Workbench, and then suppress the surfaces of the new part. You will need to include a mass definition for it, which should be subtracted from the spar mass. The mass of the dummy structure should not be very small, as this may cause numerical stability problems in the time domain calculations.
I hope this helps!