I have asked Hull City Council for details of their southern boundary along the Humber using the Freedom of Information Act. I asked:
I am interested in the boundaries of Hull, especially in the Humber. I therefore request where is the boundary of Hull UA along the Humber estuary? Does the boundary abut the boundary of North Lincolnshire or is there a gap between them?
They have answered my request. I'm not sure I can post their full answer because the email states that I should treat the email as confidential, so I'll paraphrase it:
They don't have any written records of where the boundary is.
They sent me a pdf with the inevitable, copyright OS map, which I cannot trace for OSM. It is interesting because the boundary doesn't follow the convention of being the mean low water mark, but this could be something to do with the docks. The docks used to be most of the extent of the city boundary along the Humber, so this might be why the boundary juts out into the river.
So, how to draw this onto OSM without breaching the OS copyright. Well the really good news is that the boundary looks the same as the one on the out-of-copyright map which we can trace. I now have what I need to have a go at improving some of the boundaries in the Humber.