Both sides on the cruise controversy behave, on the public eyes, as angry children on a schoolyard yelling at each other. Both grossly overstate their case and abuse the best science concerned.
The re-routing of ships makes sense. The Venetian lagoon has 2 major entrances, one on Lido that takes ships via the Giudecca canal, thus passing in front of Piazza San Marco and other major sites. The other one takes ships via Mmalocco and a dragged canal that arrives on the port via West, on open waters, without any narrow passages within the main island itself. This change is a good one, though it takes away the breathtaking views cruise passengers enjoyed from the upper decks of cruise ships. So far, so good.
The problem is that this re-routing has not placated the local NIMBYs and misleading activists. They would rather have no cruise ships whatsoever docking there if they had their way. Claims of wake damage cause by ships using other approaches to the port are bogus. In any case, there is an active industrial port in Mestre that sees plenty of cargo ship traffic which isn't going anywhere else.
Cruise operators on their hand overstate their case and push preposterous claims that without the ships the city would become a ghost town with an economy in complete collapse. They also resisted the course change with exaggerated baseless complaints about risks to navigation - the operators just don't wanted the main sight of the port approach to be preceded by side views of the industries in Mestre...
The restriction on large tonnage ships is not necessary nor justified.