The company pleaded guilty to 51 charges of breaching environmental laws between 2010 and 2015.
Southern Water has been fined a record £90 million after bosses admitted dumping sewage illegally thousands of times over a five-year period.
The company pleaded guilty to 6,971 unpermitted sewage discharges – the equivalent to one pipe leaking continuously for seven years.
Passing sentence, the Honourable Mr Justice Johnson said, of the formal 51 guilty pleas, that the company’s behaviour had been “shocking”.
He said: “Each of the 51 offences seen in isolation shows a shocking and wholesale disregard for the environment, for the precious and delicate ecosytems along the North Kent and Solent coastlines, for human health, and for the fisheries and other legitimate businesses that depend on the vitality of the coastal waters.
“Each offence does not stand in isolation. It is necessary to sentence the company for the totality of the offences to which it has pleaded guilty. But even that does not reflect the defendant’s criminality.
“That is because the offences are aggravated by its previous persistent pollution of the environment over very many years.”
Bosses deliberately painted a misleading picture of compliance to the Environment Agency which brought the criminal prosecution, Canterbury Crown Court heard.
And some of the dumping hit conservation sites, causing major environmental harm to shellfish waters.
The criminal prosecution follows a £126 million penalty on Southern Water in 2019 as a result of the company’s regulatory failings over the same period.
Chair of the Environment Agency, Emma Howard Boyd, said: “With nature in crisis, no-one should profit from undermining environmental laws.
“This sentence shows fines for environmental offences are starting to reach the same level as the highest fines for crimes in financial services and that is good.”
She added: “Like all water companies, Southern Water has a responsibility to operate in accordance with permit conditions and protect against serious pollution.
“In its deliberate, widespread and repeated offending, it has failed the environment, customers and the system of environmental laws the public puts its trust in.”
The case is the largest criminal investigation in the Environment Agency’s 25-year history and saw pollution offences from 16 waste water treatment works and one storm overflow.
Environment Minister Rebecca Pow said: “The findings in this case were shocking and wholly unacceptable. Water companies should not be letting this happen and those that do will be punished by the full force of the law.
“This fine, the largest ever imposed on a water company, is absolutely appropriate and welcomed.”
Southern Water chief executive Ian McAulay said: “I am deeply sorry for the historic incidents which have led to today’s sentencing and fine.
“I know that the people who rely on us to be custodians of the precious environment in southern England must be able to trust us.
“What happened historically was completely unacceptable and Southern Water pleaded guilty to the charges in recognition of that fact.”
He added that the fine would not have an impact on customers’ bills, with shareholders bearing the cost.