Thanks for pinning that post. I can’t get methods B or C to work with existing ferry lines though.
For method B the new pinned post mentions that you can use the data layer as a way of spotting whether these are in the database:
Long excerpt for option B
I don’t think this actually works. See for example this view of this ferry line. The line does not display unless at least one node from the way is on screen.
Likewise option C doesn’t work in the likes of JOSM which also needs nodes in view and doesn’t appear to work for iD either for the view above.
Overpass turbo does appear to catch them. Don’t know how quickly this gets updated.