WARNING The final decision on any legal right of Navigation can only rest with a court. I can take no responsibility if you do not check the legal validity of what is posted here and you get arrested or sued as a result.
This will be presented as a reverse timeline, most recent events first.
1992 - 15th July, House of Commons debate (Recorded in Hansard)
The Secretary of state for transport is asked about the protection for the public rights of access and passage to and along the Navigation during construction works for the M3. He states the government
is advised that the Navigation is in law a Navigable Waterway, and that the diversion works would enable it to be made navigable in future, and this would adequately protect the public rights1874 Hargreaves V Diddams (Court Case)
A public right to fish in the Itchen was claimed - the court ruled there was no right to fish. Even though the Navigation Act of George 3rd had created a public right to navigate the river (as a highway), the ownership of the soil (and rights to fish) remained in the owners of the adjacent land.