Venice itself has no beaches — it is an island city in a lagoon. For sand and sea you need to go to the barrier islands on the Adriatic side of the lagoon. The nearest and most accessible is the Lido, a fifteen-minute vaporetto ride from San Marco. Further south along the same chain of barrier islands, Pellestrina has a quieter, more local beach culture with a long sandy strand and a handful of restaurants.
The Lido beach is divided between free public sections and the grand stabilimenti, private beach clubs where you hire a cabana and a pair of sun loungers. The public sections are at the two ends of the island. The water is clean and warm from June through September. The island is long enough that a bicycle — available for hire near the vaporetto landing — is the sensible way to explore it.
For a longer beach day, Chioggia at the southern end of the lagoon has the beach at Sottomarina just across its bridge, with long straight sands facing the open Adriatic.