Norfolk Broads Forum now features a 
I have always been curious about this ancient right and often surprised how it is very often suppressed on the Norfolk Broads.
Navigation on Tidal Rivers
The right of navigation exists as a right of way in all tidal waters including rivers
and at common law the public have the right of passage in boats. There is also
an ancillary right to tow, and to anchor, ground or moor in the course of ordinary
navigation.
The part that always sets minds wondering is the riparian law side or perhaps in layman's terms...the moment you drop your mud weight.
It is potentially ambiguous, however, I am quite certain that boaters have quite more freedoms of movement by boats than is commonly known or broadcast.
So by all reckoning an act of parliament would be required to stop people boating or the tidal waters would actually have to completely disappear.
Extinguishment of a Right of Navigation
A public right of navigation in a river or creek may be extinguished by an Act of
Parliament or by natural causes such as recession of the sea or accumulation of
mud.