Italy has declared a state of emergency in Venice after an exceptional tide surged through the historic city, Prime Minister Giuseppe Conte said.

"The cabinet has approved the state of emergency in Venice," Mr Conte tweeted after Tuesday's devastating flooding, also announcing the release of €20 million "for the most urgent interventions".

The tide surged through churches, shops and homes, causing millions of euro worth of damage to the UNESCO city.

Sirens warning of fresh flooding rang through the canal city early today but the water level remained low compared to Tuesday's tide, the highest in 50 years.

Prime Minister Giuseppe Conte, who has called the flooding "a blow to the heart of our country", met Venice's mayor and emergency services before visiting businesses and locals affected by the tide in a speedboat.

Residents whose houses had been hit would immediately get up to €5,000 in government aid, while restaurant and shop owners could receive up to €20,000 and apply for more later, the prime minister said.

Several museums remained closed to the public today.

As authorities assessed the extent of the damage to Venice's cultural treasures, such as St Mark's Basilica where water invaded the crypt, locals were defiant.

Many stopped for their habitual coffees at flooded bars, drinking their espresso while standing in several inches of water.

Austrian tourist Cornelia Litschauer, 28, said she felt mixed emotions seeing Venice's famous square half submerged.

"For the tourists it's amazing, it's something to see. But for the people who live here it's a real problem," Litschauer said, cradling her white Chihuahua Pablo.

"It's strange. Tourists are taking pictures but the city is suffering." 

We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences

The Locanda Al Leon hotel said its bookings had suffered from the international media coverage of the flood, with some guests cancelling their rooms after seeing images of Venice underwater.

Under the arches of the Ducal Palace, a couple from Hong Kong posed for photos in the chilly morning sun.

"This (trip) was planned a long time ago so we couldn't change it," groom Jay Wong, 34, said. 

"Actually this is a good experience. It's an adventure."

Tuesday's "acqua alta," or high waters, submerged around 80% of the city, officials said.

Only once since records began in 1923 has the water crept even higher, reaching 1.94 metres in 1966.

Former Prime Minister Silvio Berlusconi arrived for a private tour of the damage sustained to the basilica, while rival leader of the Italian right Matteo Salvini was due to drop by for the same on Friday.

French tourist Manon Gaudre, 22, said seeing Venice submerged was a "unique experience".

"The damage it's causing to monuments and the people is worrying," she said, wondering if climate change was to blame.

Many, including Venice's mayor, have blamed the disaster on global warming and warned that Italy - a country prone to natural disasters - must wake up to the risks posed by ever more volatile seasons.

"We need to be resilient and adapt. We need a policy that looks at the climate through completely different eyes," Environment Minister Sergio Costa said.

Mayor Luigi Brugnaro has estimated the damage to Venice at hundreds of millions of euro.

The Serenissima, as the floating city is called, is home to a mere 50,000 residents but receives 36 million global visitors each year.

A massive infrastructure project called MOSE has been under way since 2003 to protect the city, but it has been plagued by cost overruns, corruption scandals and delays.

"This engineering solution that will end up costing nearly six billion euro has got to work," Transport Minister Paola De Micheli said.

The plan involves 78 gates that can be raised to protect Venice's lagoon during high tides - but a recent attempt to test part of it caused worrying vibrations and engineers discovered it had rusted.