The survey, answered by almost 1,300 workers, found employees had been attacked with machetes, suffered broken jaws and been deliberately splashed with raw sewage while at work.
Some GMB members made additional testimony. One worker said they had been assaulted by minors while their parents stood laughing, while another said he had become used to verbal abuse and being threatened over the years.
Employees also said they do not feel safe wearing clothing bearing water company logos or working alone at nights.
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Gary Carter, GMB national officer, said: “No one should go to work and face abuse.
"But this situation is horrifying, the negligent actions of water bosses in allowing sewage dumping to rocket has exposed their own workers to physical and verbal violence.
“In many cases, water bosses are actively discouraging the reporting of spills, which is a complete dereliction of duty.
"GMB demands a zero-tolerance approach to the abuse of water workers and calls on water shareholders to fork out for the desperately needed infrastructure to stop record sewage spills.”
Questions answered by just under 700 workers in the water sector found 22% had seen unreported spills in the past year.
Similarly, 22% had either been encouraged to underreport sewage spills, or knew a colleague who had.
Responding to the survey, a spokesperson for economic regulation body Ofwat told HR magazine: "In the water sector, tens of thousands of people work hard to ensure the provision of safe drinking water and the proper treatment and disposal of wastewater.
“It is totally unacceptable for anyone to experience any form of abuse or violence due to their employment. Any such incidences should be reported to the relevant authorities, including employers, and the appropriate safeguards put in place."
The news comes after the Environment Agency launched a new portal for water company whistleblowers to safely report serious environmental wrongdoing by their employer.
Alan Lovell, chair of the Environment Agency, said: “We share the public’s disgust with sewage pollution and know there’s always more that can be done to protect our waterways.
“This new whistleblowing portal allows workers to raise their concerns and we encourage people to come forward, knowing any information will be treated in confidence and with sensitivity.”
Elizabeth Gardiner, chief executive at the whistleblowing charity Protect, told HR magazine that there are low numbers of environmental whistleblowers.
She said: “It is surprising and alarming that there continue to be low numbers of people blowing the whistle about environmental issues.
“Out of the thousands of whistleblowing calls we receive each year, only a handful relate to the water industry.”
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She said whistleblowers are the most effective way of recognising risks early.
“Serious harm to the public and to the environment can be averted by heeding whistleblowers' early warnings. It’s essential that water workers are aware of their organisation’s whistleblowing procedures and their right to raise concerns.
“Employers need to remember that whistleblowing is good for business too: responding effectively to concerns can save money and reputations and increase staff loyalty and productivity. It is vital that organisations have policies and good speak up cultures in place to facilitate whistleblowing, to conduct investigations and to protect those who raise concerns.”
