Currently vaporetto routes marked as ferry routes, and they has no clear route number mark on map. Typical ferry routes outside Venice - between two stops only. Venice vaporetto system has much more complicated routes, but no reasons to make more complicated route mark to all world simple ferry routes. May be better to mark vaporetto system as bus routes?
Bus routes already has correct drawing on maps like publictransportmap.org, and great interactive using in apps like OSMAND.
Italy isn’t my area, but I thought, if local osmers will agree with advantages of my suggestion, they can apply it.
To me, bus routes are for busses. As the vaporetti aren’t busses, I would not use them. For example there are similar public services in other places which are also mapped as ferry routes like these:
They are very similar, also operated by the local public transport company and operated with similar frequency, pricing, etc.
If there is no route number on the map (Berliners have “solved this” by adding the number into the name), the better approach for me would be to ask the map style developers for support of ref.
On a side note: this forum is not used very much. If you are interested in more extensive feedback you should try to send an email to talk-it.