Just seven of Northern Ireland's beaches receive a recommendation in this year's Good Beach Guide, it was announced today.
The Marine Conservation Society (MSC) recommendation was given to only 30% of the 23 beaches tested - down from the 43%, or ten beaches, which hit the mark last year.
One beach - Ballyholme in Bangor, Co Down - failed the minimum legal water quality standard altogether, although that was a drop on the three failures of 2008.
Twelve more beaches received a 'basic pass'.
The society said Northern Ireland continued to suffer from a combination of storm run-off carrying material like fertilisers and animal waste from farmland into the sea, and years of under investment in the sewage infrastructure.
It added: "Heavy summer rains caused a significant dip in water quality but the province is now undergoing a £420 million sewerage investment programme which MCS hopes will start bringing improvements in the near future."
Across the UK as a whole standards dropped with 16.5% less beaches recommended for their excellent water quality and an almost 50% increase to 78 in the number failing.
MCS Coastal Pollution Officer Thomas Bell said "Today's results reflect last summer's heavy rain which swept waterborne pollutants like raw sewage, petro-chemicals and farm waste into rivers and the sea.
"MCS is now recommending 25% fewer beaches than three years ago and we're becoming concerned that the existing infrastructure for handling storm pollution may not be up to the job."
Northern Ireland Water which is responsible for water quality in the province accepted it had work to do to improve matters.
Angela Halpenny, it's wastewater regulations manager said: "While we are taking steps to improve our wastewater systems and progress is starting to be seen, we do acknowledge that there is still work to be done."
She said NI Water welcomed the MSC's comments that Northern Ireland's results continued to suffer from agricultural land run-off and underinvestment in the sewage system.
"Weather plays a major role in the quality of bathing water. During periods of heavy rainfall, the sewage system can overflow, which it is designed to do, to alleviate the sewer flooding.
"Last summer's high rainfall increased the frequency of overflows and run-off from agricultural land and urban areas," said Ms Halpenny.
She said improving the wastewater infrastructure was a major priority and by next year the water company planned to have invested £490m in improvements to the sewage network and wastewater treatment works.
The Ballyholme failure was "unfortunate" she said, but added a £3.5m project to improve the sewage network in the area had begun late last year and would bring significant benefits which would "contribute to improvements in bathing water quality in the area."