Written complaints made by customers to their water providers have fallen for the sixth year in a row in England and Wales, figures from a watchdog show.
There was an 18% fall in complaints in 2013-14 compared with the previous year, to 123,218, the Consumer Council for Water said.
This was the lowest level since the watchdog was formed in 2005.
However, it said that this momentum could be lost if water companies failed to deliver affordable bills.
Billing and charges still account for the highest proportion of complaints, some 57% of all gripes.
Four companies - South West Water, Affinity Water, Severn Trent Water and Dŵr Cymru Welsh Water - reported an increase in complaints.
Southern Water and South East Water, while reporting drops in complaint numbers, were described as remaining "too far adrift" of the rest of the industry.
Darren Bentham, Southern Water's chief customer officer, said: "While our performance in 2013/14 saw a big improvement, we are still lower down the results table than we want to be - and where our customers want us to be. However, we are continuing to make changes which ensure we focus on our customers - from training, to new systems and an improved website."
Steve George, customer services director at South East Water, said: "Although our focus is to prevent problems, when things go wrong for our customers we are always sorry and we endeavour to fix things as quickly as possible."
In August, regulator Ofwat proposed that household water bills in England and Wales should go up less than the rising cost of living in the next five years.
It proposed that bills should be an average of 5% lower, before inflation was applied, by 2019-20.
Although telephone complaints were also falling at the same time as written complaints, these trends could reverse if prices were not set at the correct level, the Consumer Council for Water said.
"Affordability remains a huge challenge for the industry with one in five customers telling us their water bill is not affordable," said Tony Smith, the watchdog's chief executive.
"Water companies and the regulator Ofwat must deliver prices for the next five years that customers can afford and find acceptable or risk a backlash from struggling households."