In message

, at 18:26:56 on Wed, 6 Jan 2016, bob remarked:
No, it also depends on things like the width of the river and whether
there's enough time for the tidal waters to get that far upstream before
the tide turns.

The rivers in some parts of Sussex are tidal further inland to a
greater extent than some people may realise. This encouraged
connecting Waterways in the area. at one time Petworth, Midhurst and
London could be reached by water,The latter route is better known
especially since two wine drinking thespians featured it recently
between crashing thier boat but Petworth,and Midhurst and some minor
places near them seem unlikely places to reach by boat now.

There's no dispute that Cambridge, 40 miles inland[1], could be reached
by sea-going craft; but that doesn't mean it's tidal.

Where I live, about 30 miles inland on the Great Ouse (which leads
upstream to the Cam), there are numerous broad-beam boats which have
clearly crossed the North Sea to get here.

There are plenty of rivers that are accessible by seagoing craft a long way
above their tidal limits. The Rhine, for example, is navigable by ships
capable of coastal navigation as far up as Basel, which is nearly 250 m
above sea level and definitely not tidal.

That's right. Exactly the point I was making.
--
Roland Perry