From where the cruise ships dock it's a 15 min walk just to St Mark's Sq. Up lots of steps and over bridges. But it's not far. You can get a vaporetto (water taxi) from one of the stands, and go up the Grand canal to the station. If you have luggage, there is a ramp at the east side of the station, don't need to drag luggage up the stairs. Ask which vaporetto to get. Most will go to the station, I think. The locals pay 1 Euro a ticket. Tourists pay more. If you want to use the vaporetto a lot, you can get a pass.