There are canals flowing into the lagoon, but it is difficult to square these waterways with the glories of Venice. Grey-brown swirls sweep out of the narrow channels into the shallow, weed-filled water of the lagoon.
This is all good, clean waste water processed according to the dictates of national and EU laws, say the chemical giants producing PVC, polyurethane and solvents by the shores of the lagoon in Porto Marghera.
But their claims of environmental correctness have failed to convince Italy's new generation of determined young magistrates.
For the first time in its four-decade history, the heavy industry of Venice is being called to account for what the city's deputy mayor, Gianfranco Bettin, recently called "the holocaust in the lagoon".
Last week, a judge served a confiscation order on the waste water outlet SM15, the biggest in Porto Marghera and the one into which the detritus of two of Europe's largest chemicals producers - Italy's Enichem and EVC, a subsidiary of ICI - flows. The order was lifted later, but only after judges said they were satisfied the industry had cleaned up its act.
In his confiscation request, the public prosecutor Luca Ramacci had included charts which showed that the water leaving SM15 contained levels of carcinogenic chemicals and heavy metals up to three times over the legal limit.
Greenpeace's Venice expert Fabrizio Fabbri, says this only tells half the story. "The water pollution levels are appalling, but what lurks in the sediment is just as worrying," he said.
"There are still high levels of DDT, which they stopped producing years ago, and PCBs, and of course dioxin. All these substances break down very very slowly. They'll be there for tens, if not hundreds of years."
The Venice health authority, in a recent report, denied that any of these potentially fatal chemicals had entered the food chain. It saw no reason to upset the flourishing seafood industry in the lagoon with what they call unnecessary scare-mongering.
"If what they say is true, I'd call it the Miracle of St Mark," said a sceptical Mr Fabbri, referring to Venice's patron saint. "If dioxin at these concentrations gets into the food chain everywhere else in the world, it's difficult to imagine why it shouldn't do so here."
The local health authority has also consistently denied that pollution in the lagoon presents any health hazard for people living and working there.
But in a trial which began in March, Enichem was ordered to pay 63 billion lire (pounds 20m) in compensation to the families of 150 petrochemical workers in the area who have died of tumours.
"The incidence of cancer in the towns around the lagoon is well above the national average," said Greenpeace's Mr Fabbri.
Mr Ramacci says that the ostrich-like attitude of the local authorities to the problem of the water surrounding one of the world's most beautiful cities is the result of "illicit connivance" between the people who are responsible for checking the state of the lagoon, and the directors of the companies in the Porto Marghera industrial complex.
The city's deputy mayor admits this, but says that such connivance is a thing of a past. He blamed it on the policies of the Socialists, who, until their political fortunes collapsed amid corruption scandals earlier this decade, were the principal political force in the Venice region.
Inquiries have shown that the chemical industry certainly lined politicians' pockets, and that this left them free to pollute the lagoon with impunity. "They were used to having carte blanche," Mr Bettin said.
He now sits on a committee with representatives from local authorities, industry, and unions which is studying each plant in Porto Marghera on a case-by-case basis, and halting any activity which appears to endanger the lagoon.
But there is still opposition to the attempted clean-up of the water. The chemical industry was furious about the confiscation order, saying the move would result not only in the closure of Porto Marghera plants but of well over 50 per cent of Italy's entire chemical industry, which relies on supplies from the Venice region.
The unions blocked roads and railway lines in protest, too, protesting that the jobs of about 8,000 people employed in Porto Marghera's chemical sector were at risk.
The environmentalists complain that piecemeal confiscation orders will achieve little.
"If Porto Marghera is to go out of business - and it will have to if the Venetian lagoon is to be saved - it will be due to the global decline in heavy industry, not to a confiscation order," Mr Fabbri said.
But Mr Ramacci sees light on the horizon after decades of neglect: "Industry has finally realised that power has been taken out of their hands, that there are people determined to put a stop to 30 years of destruction. The holocaust is over. Now we have got to mop up the damage."Reuse content