Water quality on five NI beaches fails to meet the mandatory EC standard, the Good Beach Guide has said.
Twenty-seven beaches in NI were monitored for the survey
The Marine Conservation Society said a major problem was poorly treated sewage discharged near the province's beaches.
Northern Ireland is the only part of the UK where raw sewage from large towns is run directly into the sea.
However, the Water Service said it planned to invest £420m over the next three years to improve waste water treatment and sewerage works.
Six beaches were recommended, based on water quality and sewage tests.
The recommended beaches were Magilligan, two at Tyrella, Warrenpoint and Cranfield.
These beaches met the criteria of the "highest water quality and adequate treatment of any continuous sewage discharges".
The five beaches which failed to meet the mandatory standard were: Castlerock; Carnlough; Ballygally; Ballyholme and Ballyhalbert.
The society "wouldn't advise" swimming at these beaches as infections such as gastroenteritis were "more likely".
Castlerock and Ballygally were recommended beaches in last year's survey.
The society said Northern Ireland could more than double its number of recommended beaches "if 12 bathing sites were not subject to poorly-treated point source sewage discharges".
Waste water treatment
Twenty-seven beaches in the province were monitored for the survey.
Overall, Northern Ireland came seventh out of nine regions in the survey with 22.2% beaches attaining the top award, down 7.4% on last year's figures.
The south west of England came top with 75% of its beaches being recommended.
The Northern Ireland Water Service said it admitted the reduction from eight to six recommended beaches was disappointing.
However, it pointed out that the Marine Conservation Society used stricter - non statutory - criteria to determine water quality and said almost 80% met EC standards last year.
It said that of the planned £420m investment to improve waste water treatment and sewerage works, £100m of that would go towards upgrading waste water treatment working in coastal areas.