The best way to visit Venice is on foot. Alternatively, you can take the “Venetian bus”, the waterbus (vaporetto).
For visitors the standard one-way fare of the waterbus is 7,5 Euro. Thus, if you decide to use it during your stay in Venice it is better to buy a 3-day travel card (40 Euro) or a 7-day travel card (60 Euro).
You can check lines and timetables here.
To get more, detailed info, check here.