Tides have surged six feet in the Venetian lagoon flooding the town's ancient squares and builldings.
Some schools were closed as a precaution as the waters poured in.
Tourists and Venetians donned high boots and have taken to temporary raised walkways to slosh through the high water that has hit much of the city.
Venice's tide forecast office said the water level peaked at 1.27 meters (4ft 3in) on Tuesday morning but warned that an even higher tide was forecast for after nightfall - peaking at around 6ft on the late high tide.
The high water invaded cafes, stores and other businesses. Sirens warned people in Venice of the rising water, and as a precaution, authorities closed nursery schools.
A top tourist attraction, the Ducal Palace, just off St Mark's Square, tweeted that it was "open today, despite the exceptional tide", and advised visitors to use the raised walkways leading to its entrance.
Many hotels keep disposable knee-high plastic boots handy for tourists. Venetians' wardrobes often include over-the-knee rubber boots.
The high water, known locally as "acqua alta", was amusing for tourists and a nuisance for residents going about their business, but levels were lower than the 1.94 meters (6ft 4in) in the devastating November 1966 flood.
But even lower levels of the salty high water over the years take their toll on the city, eroding foundations of homes, businesses and city buildings.
Bad weather is continuing to dog Italy, with no real let-up forecast for several days.
In Policoro, a southern town in an area known for its ancient Greek ruins, a whirlwind ripped the roofs off two homes, but the occupants inside escaped injury, Italian news reports said.
In the same region of Basilicata, areas of the tourist town of Matera, famed for its Sassi former cave dwellings, were flooded after heavy rains.
