From November 2014, no more
In early 2012, right after the Costa Concordia disaster, a new law was enacted in Italy to force big ships not to get closer to Italian shores. However, the new law was not applied in Venice fearing it might have a negative impact in local tourism industry.
The lagoon surrounding Venice is a UNESCO site, and it is a matter of fact that it might be damaged by the frequent transit of huge vessels. Venice is one of the world's most important cruise destinations, with more than 650 ships ships passing through the city annually.