Customer complaints about water firms in England and Wales rose by 11% in the last financial year, the watchdog said.
Complaints about the water quality from taps fell
The Consumer Council for Water said customer trust was "getting stretched" for reasons such as above-inflation price rises and drought restrictions.
CCWater replaced consumer body Water Voice last October. Together the two bodies received 12,359 complaints in the 2005-06 financial year.
United Utilities and Severn Trent Water saw the biggest increase in complaints.
CCWater received 1,138 complaints about United Utilities, which serves the North West in the six months up to 31 March - a 41% rise from the same six months last year.
Meanwhile, Severn Trent, which serves more than 8m customers across the UK, received 1,100 complaints - representing a 55.6% rise.
United Utilities said "teething troubles" with a new billing system had prompted a rise in complaints.
Gary Dixon, who is responsible for customer service at the firm, said the introduction of the new system had been a "major project" involving the transfer of millions of customer records and training for more than 1,000 employees.
"This system has now bedded-in and the number of complaints to United Utilities and those being investigated by CCWater has fallen since March," he said.
Discounting these two firms, over the same six months customers made 4.5% fewer complaints about all the other water and sewerage firms. The number of complaints about the quality of water coming out of the taps also fell significantly.
Dame Yve Buckland, chair of CCWater, expressed concern at what she called a "significant" rise in complaints.
"From our research we know that consumer trust is getting stretched. They expect to see service improvements and reliable supply alongside returns to shareholders," she said.
Most complaints were about unmetered bills, which saw a 66% rise than the previous year, and about metered bills, where complaints rose by 40%.
Customers were also concerned over delays in connections to the mains - will complaints up by 35%.