The second in line to the throne donned a wetsuit and took a dip with children in a swimming pool to celebrate becoming the president of the British Sub-Aqua Club.
The 32-year-old royal, looked like he was enjoying himself in the water, giving the "ok sign" to his fellow divers.
The Duke of Cambridge has taken over from his father Prince Charles, who also attended the event at the Oasis Sports Centre in London's Covent Garden.
Eugene Farrell, the chairman of the BSAC, told guests that in his diving days the Prince of Wales was "buff", referencing a shirtless photograph taken of Charles in the 1970s.
Prince William then had to explain the meaning of the word to his father and that it was a compliment. He reportedly said: ""Pa, you should be pleased!"
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