The royals were taken on by a host of celebrities at the star-studded sailing regatta, including comedian John Bishop (for ActionAddiction), BBC presenter Katie Thistleton (who was supporting Place2Be) and Olympic Champion rower Helen Glover (who was representing the Anna Freud National Centre for Children and Families).
The Duke and Duchess of Cambridge couldn't keep the smiles off their faces as they - and their celebrity rivals - competed for the King's Cup trophy.
Kate was the picture of concentration as she steered her Fast-40 boat.
Unfortunately, Kate's best wasn't good enough, as she received the wooden spoon.
The royal couple finished in third and eighth place, respectively.
Prince William and Kate shared a laugh as they attended the award ceremony, where Kate was awarded her wooden spoon.
The Cambridges congratulated winning skipper Bear Grylls as he was presented with the King's Cup trophy.
After lifting the trophy, Grylls made a startling statement about Prince George.
"And also Prince George, your first ant you ate today. And that is a great moment. Well done you."
Earlier in the day, Prince George was seen watching his parents with his sister in a support boat along with their grandparents Michael and Carole Middleton.
The pictures showed that it was a fun day for the Royal family at the King's Cup regatta.
Princess Charlotte drew laughter from the crowd when she stuck her tongue out at them, instead of waving.
For more Royals news, pick up a copy of this month's Royals magazine.