Acqua alta (high waters) is a natural phenomena that hits Venice several times a year. A conjunction of high tide and strong winds push water into the Laguna that surrounds the city. Ultimately it floods the streets of Venice for a few hours. Though Acqua alta always existed, it has been magnified over the last century by human intervention (lack of drainage, too many motor boats, …) and Venice is slowly drowning. Well, not for long, since major works are in progress to stop the process. So if you'd like to discover the magic of Plaza San Marco being flooded, head to Venice soon. You'll just have to get lucky with weather conditions.
As for the picture, taken in Plaza San Marco, it shows how Venetian deal with the problem. That is by elevating temporary walkways over ground level. (click on picture for high res)
Leica M9 with 35mm Lux Asph at F1.4, 1/1000, ISO160