About 16,000 homes and businesses have been advised to boil water before drinking it until further notice.
SWW said it was draining the Hillhead reservoir and the wider Alston area of Brixham was still being investigated as a potential cause for the outbreak.
Kayley Lewis, 33, said she and other members of her family had been suffering from cryptosporidium symptoms since 5 May, adding she had lost about 6lb (3kg) in weight in six days.
She said her 13-year-old son had become so ill, they had had to rush him to hospital after blood was found in his vomit, which she said was “scary” to deal with.
On Tuesday, SWW officials said there were no issues with its treated water and tap water was safe to drink, but 24 hours later told residents to boil their water after traces of cryptosporidium were found.
Ms Lewis said initial advice and the later U-turn summed up the “disgusting” way SWW had responded.
Ms Lewis said: “They took samples from my neighbours and even said to them it’ll be 24 hours until we get the results.
“But two hours later they mentioned on social media that it was fine to drink the water in Brixham when clearly it wasn’t OK to drink.”
