The announcement was made at the Better Beaches Forum held in Lisburn.
Angela Halpenny, NI Water's Head of Environmental Regulation, welcomed the results.
She said: "Protecting the environment is an important part of our business and we play a key role in delivering 570 million litres of drinking water and collecting and treating 340 million litres of wastewater every day.
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"NI Water is delighted to hear that all 26 designated bathing waters in NI have passed in 2018 and is proud of our role in helping achieve this.
"Delivery of recent sewer network investments worth around £11.5 million in the Bangor area have undoubtedly contributed to ensuring the bathing water in this area is no longer of poor quality.
"We cannot become complacent however; we must continue to push forward with further investment to ensure continued improvements are made. Our business is essential to support a thriving population, a growing economy and a flourishing environment.
"To continue to do so however, we must be adequately funded. NI Water is not immune from public expenditure cuts and uncertainty over funding. This places progress on efficiencies at risk and could result in tangible impacts on service delivery, the local economy and the environment."
