If you are in Venice and feel like you want to go to the beach, you can! To do this you must take the ferry number 1, direction Lido. It will cost you around 7 euros to go and come back. If you take the ferry (or vaporetto, how they call in Italy) in San Marco it will take you 20 minutes to reach Lido station. If you take it in Rialto, it will take you almost one hour.
Lido has a different environment because over there they have streets, cars and bicycles and at the same time way less tourists than Venice. To reach the beach you need to follow straight Maria Elizabetta street that starts in front of the station. In the end of this street you can see a white construction and after crossing that, you are in the beach.
If you are up to spend a bit of money there are plenty of services offered such as paid toilet, cabins, tables, beach umbrellas and other conveniences.
If you are low on budget and just went to enjoy the free beach, bring your own towel. I went with Bernardita, a Chilean girl that I met in Prague and after hosted me in her place in Venice, trough Couchsurfing.
The water was cold, but even tough I had a swim. It was cleaner than the famous Barceloneta beach in Spain. The beach has plenty of shells as well.
After enjoying the beach I went back using the same street and did a stop to have a 5 euros hamburger in a little kiosk in the street. It was a good price for Venice.