In message , Dave Arquati
writes

I'm told that the tidal nature of the Thames has an adverse impact on
the cost of running the services, although I admit to some trouble
understanding this.

In addition to the points already made, the strong tides and big
differences between high and low water require expensive, substantial
piers and relatively slow docking procedures.

In contrast, the very low tide-fall in Venice (normally only a few
inches) means that piers can be very lightweight and docking takes just
a few seconds, resulting in much faster and more efficient services.

--
Paul Terry