Acqua alta (high water) has become a fact of life in Venice. The lagoon water level occasionally rises above the level of the squares and streets, flooding them. This can happen several times a year, at irregular intervals, usually in the colder months. Acqua alta usually lasts a few hours and coincides with high tide. You'll see raised walkways in side alleys ready to be pulled out when acqua alta hits. When the city begins to flood, sirens will sound to warn residents and businesses. If you speak fluent Italian, tune into news programs since their predictions of the times the flood begins and ends are usually on the spot. Normally, the tide rises and falls in six-hour cycles.
You can get an acqua alta map at the tourist offices either at the railway station or St Marks. This will show you the higher, dry routes and the ones with walkways set up during the various flood alerts. There is a tide measuring station at the Rialto vaporetto piers, and a noticeboard at the base of the Campanile in the Piazza San Marco that shows a live tide reading and predictions for the next few days.