"In 2025, the North Sea reached an average temperature of 11.6C, the highest value in the BSH data series since 1969," Tim Kruschke, head of the BSH's climate team, said in a statement.
The agency said that the Baltic Sea also "approached record" temperatures last year.
The BSH said the annual average surface temperature in the Baltic in 2025 was 9.7C, an increase of 1.1C compared to the long-term average from 1997 to 2021.
"This made 2025 the second-warmest year for the Baltic Sea since the BSH data series began in 1990," the agency said, adding that "only 2020 was warmer".
Sea temperatures are rising globally due to climate change.
The rate of ocean warming has more than doubled since 1993, according to the UN's IPCC climate expert panel.
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