The Pennon Group, which owns the utility, said the parasite outbreak contributed to a nearly £40m bill for the company.
Customers who were affected by the boil water notice, and those who were unwell, were offered compensation by South West Water.
Lisa Horswill, who was among those affected, said the conviction of the water company was “bittersweet because it’s been really unpleasant for some of us trying to fight for justice”.
“The fines are only going to be passed on to us and that’s not right,” she said.
Her partner Kris said any fine “would not be enough”.
A South West Water spokesperson said: “We can confirm customers do not pay for fines.”
The inspectorate said the fine was “unlimited under the legislation, however, it will be up to the judge to determine the most appropriate fine”.