It is very important to validate your ticket while traveling. Tickets need to be validated for traveling on bus, train or boat.
Bus tickets: The bus tickets can be validated once you get in. This is the case for any bus journeys in Italy.
Train tickets: The train tickets need to be validated before entering the train. There are yellow machines on the beginning of every platform ready for use. In case of emergencies or when you forget validating your ticket look for the train staff. In this way you will avoid your fine.
Boat tickets: Tickets must be validated before going on board. The machines are  present on the vaporetto platforms. Hold your ticket within 6 cm of the circular panel for three seconds. Once you see a green light and hear a beep as confirmation that the ticket has been validated.
Not validating the ticket gets you a fine because that ticket can be used on another trip. Unlike bus tickets in the UK that are already validated, Italy has a different system so it is very important you remember this.
Prices for single boat trips are quite high, so consider buying a 12-hour to 7-day Travel card. In case you are staying for a long period than even a Venice card is the best option.