See one of the huge cruise ships sail through Venice. In 2000, they went up and down the Canale della Giudecca 400 times, in 2011, 1, 308 times. The canale is officially categorised as open sea and comes under a central government authority, the Autorità Portuale di Venezia, over which the mayor has no authority.

http://youtu.be/lE6hP2G0yVc

The port in Venice is currently being expanded to accommodate more of them. There are huge economic interests involved and the mayor has little say in this matter as the port, and the Canale della Giudecca down which the ships sail, come under the Venice Port Authority, an organ of central government, whose powerful president, Paolo Costa, believes firmly that economic development takes precedence over conservation issues.