Across the lagoon from Venice is the island of Lido, long renowned as one of Italy's best beaches and home to the first international film festival in the world (the celebrated Venice Film Festival, which started in 1943). The main strip is the Gran Viale, where you'll find the cenrally located public beach. If you're staying on the island, your hotel may have a private beach; otherwise, this is your best option, despite it getting quite crowded on the weekends.
Alberoni is a protected area on the island that has seen far less development, and stands in contrast to Lido's chic demeanor. If you really want to escape the shambling tour-zombies, then make your way out here to an expanse of sand dunes and clear waters. In general, the Lido is a pretty tame time and nothing like the brothel-riddled strip of a hundred years ago, yet still proves to be a nice diversion from classic Venice.
Note:For those of us who could use some gambling, the Venice Casino on the north end of the strip is a mainstay.