On its journey to the North Sea, the River Thames runs through Henley, a picturesque middle-class town about 35 miles west of London. The river is central to the town’s identity, which is known for its annual royal regatta and for being the location of the first boat race between Oxford and Cambridge Universities.

Long a source of pride, the river is now a source of alarm.

“In the last two years, it’s just got worse and worse and worse,” said Laura Reineke, 51, a Henley resident who started a local swimming group, the Henley Mermaids.

The water is “murkier, and if you do see vegetation on the bottom, it’s covered in sewage fungus,” she said. While swimming in the Thames, she added, the group has also seen sanitary products caught in the plants, colostomy bags and what appear to be excrement marks on swans.

Nearby, wastewater treatment plants for Thames Water, England’s largest water company, sometimes dispose sewage into the river and local streams, drawing complaints from Henley residents.

Thames Water is not alone. The 10 water utilities in England and Wales, which were privatized in 1989 during a wave of deregulation and free-market liberalization, have become a target of public ire over polluted waterways and rising household bills.

The number of people getting sick from the water is growing, according to data collected by campaign groups and hospital admissions. Surfers Against Sewage, a nonprofit group that tracks water quality around Britain, said more than 1,900 people reported getting ill after entering the water last year, nearly triple the number from previous year. But this was likely just “a glimpse into the true scale” of the problem because it included only reports submitted to the group, it said.