I have travelled on a couple of occasions, years ago, by canal through much of the Netherlands, on a converted dutch barge. One such does make a very pleasant mode of transport. There are many different types and sizes; the beam is typically 4m - 6m, which is fine on almost any river and on the wider canals which are the norm in continental Europe. I suspect that some of the older canals would not, however, accept the widest dutch barges. Although dutch barges are not uncommonly seen in the UK, they are confined to rivers and the wider canals, although they might venture into a short stretch of narrow canal up to the point where a lock or bridge became too restrictive, perhaps for mooring or residential purposes.
Yes, the 'sub' was clearly intended for continental travel - after crossing the Channel, no mean feat in itself for a 'narrowboat' - and I doubt that it would have any problem navigating the canals in France or elsewhere. If mainly intended for UK (touring) use, I am sure it would not have been built as wide as its sphere of operation as built is distinctly limited.