@Nick-Carthew Skip works fine in the web app, but it doesn't help in the Navigation app. When the route is loaded in the Navigation app, the route is recalculated without the skip.
And the Nav app behaves the same as the web app. The ferry is avoided unless "Avoid highways" is enabled.
Working this out in my head, I guess the thing to do is to use skip to include the ferry when "Avoid highways" is disabled, complete your route with expanded waypoints to force routing on highways, and then enabled "Avoid highways" in the nav app to include the ferry.
That's kind of convoluted, but I think it would work.