As far as I can tell, there aren't currently any ships that can travel on both open water and waterways. But it would seem that this would be there would be some real-life instances of this... are there? Should there be?
All ships without way constraints(MUST USE/MAY USE) can use open water and Wide rivers (Waterways) but not Rivers/small rivers/canals(Small waterways).
The largest rivers do not have the prohibitive constraint set, allowing ocean going ships to use them.
Good to know... thank you both for stating the logic so clearly.
I guess I'll have to play around with the map generation settings a bit; I haven't seen the wide rivers occur very frequently at all.