I look at the angle of the chains at the stern of the ferry and the flow rate, if the angle looks shallow wait till it's docked, if the angle looks steep pass in the centre of the gap, if the ferry is way off line wait till docked.
If you are close by on a running tide you can't simply stop and wait if the ferry takes off, getting past quick is probably the best option but hitting the throttle usually pushes the spinny bits lower towards the chains.