Shuttle bus from Venice to Venice (city)
There's no traffic allowed in the center of Venice, so it's impossible to get there by bus or car. Some cruise lines offer a shuttle bus to the People Mover station or Piazzale Roma. From there you can continue on foot or by water transport.
Sometimes cruise lines also offer a shuttle service by boat from the cruise port to Piazza San Marco. This is a fun way to go to the city center.
TIP: Unless the shuttle is free of charge, you should check the price in advance. There are several other options such as a water taxi, walk, an Alilaguna boat or a canal bus (Vaporetti).