Built only a few feet above sea level, Venice has always enjoyed a close relationship with sea. Twice a day, tidal forces fill and empty the lagoon, moving saltwater through the canals. On certain occasions, however, when a combination of wind and high tides grow more aggressive, the sea can begin to overrun its normal bounds, a time the Venetians call acqua alta.
I remember the first time I noticed the phenomenon: my wife and I were enjoying a romantic stroll through Piazza San Marco after nightfall. Cafe musicians were playing on the outer edges of the square, and as we stood listening, I noticed that a puddle in a low point of the square was expanding. Sea water had reversed up the storm drain, and was slowly lapping over the square marble stones.
Since that initial introduction to the phenomenon, I have experienced far greater inundations in Venice. I know now that when acqua alta is forecast, the workers who pilot the garbage boats around the city are redeployed, sent out to erect elevated walkways called passerelle. These structures are set up along the main routes through the city, making it possible to stroll in safety.
With a very high tide, waves will begin to break over the southern edge of the Piazza San Marco, sending water washing across the stones.
Recommended dress for an acqua alta? You couldn't go wrong with sturdy rubber Wellington boots. On the other hand, I found that shorts and sandals worked as well. If you don't mind wet feet, immersion affords some interesting angles for photography. Alternatively, you might find someone to carry you across the flooded piazza...