Venice sits in a silty lagoon, the largest natural lagoon in Italy. Changing currents and shifting sands have actually kept Venice safe from marauders over the centuries. Those poles you see in the Venice lagoon mark the channels--take them away and you'd be lost and likely to run aground, especially in a big boat.
The problem today is that various phenomenon, including normal tidal flow from The Adriatic, rising seas, silt, the jetties and shipping channels the British created for large boats in the 19th century, and the undermining of islands due to water and methane extraction, have all contributed toward the increase in frequency of flooding in Venice, called acqua alta, or high water.
The flooding can sometimes be dealt with by wooden walkways and ramps. At the worse times Venetians will be warned to stay home, as in November of 1996, when the city was submerged by almost 2 meters of water. Venice now expects about 60 occurrences of acqua alta per year.
Acqua Alta Warnings
Venice residents are warned of acqua alta forecasts by concealed speakers that emit sounds which correspond to different levels of flooding. Newspapers carry predictions as well. Residents can get SMS warnings. It's best for the tourist, especially in spring and fall when the probability of Acqua alta is greatest, to inquire at their hotel if flooding is to be predicted.
What can you do if it's going to flood? Lots of stores in Venice sell the rubber boots that residents wear when high water is predicted. It's really about all you can do.
Acqua Alta Forecast
The City of Venice publishes graphical tide predictions on the web: ISTITUZIONE CENTRO PREVISIONI E SEGNALAZIONI MAREE. You can also get notified of acqua alta via SMS message if you have a cell phone that works in Italy and can read enough Italian to fill out the form.
Acqua Alta Video
I've prepared a short video on the Acqua Alta in Venice.