The cheeky 4-year-old was passing by a window with her mother, Kate Middleton, when she stopped to look out at the crowd.
A host of other celebrities also took part in the eight-boat race, which was held a day earlier than planned due to Friday's bad weather forecast.
The boring weather did not stop the couple sporting shorts for the two races but a royal win was not to be, with Grylls winning overall.
Unfortunately for the Duchess, she was awarded the wooden spoon for her yacht's poor performance, but it's safe to say she got the last laugh when her daughter made her thoughts on the matter known (we can only assume, of course) at the winner's ceremony.
Kate was competing on behalf of the Royal Foundation and was joined by comedian John Bishop for Action on Addiction, children's TV presenter Katie Thistleton for Place2Be, and British Olympic rower Helen Glover for the Anna Freud Centre.More news: Man City and Juventus confirm Cancelo-Danilo swap in €65m deal
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Scroll on to see the Cambridge family's nautical style at The King's Cup 2019 regatta.
William's boat finished the race third while Kate came in seventh in the first race.
The Duchess was sadly disqualified from the second race on Thursday for starting too early. However, William did place higher than Kate, giving him bragging rights around the palace!
William was racing on behalf of Child Bereavement UK, and his charities, Centrepoint, London's Air Ambulance Charity 30th Anniversary campaign, and Tusk were represented by Fara Williams, historian Dan Snow, and Grylls respectively.
It's been reported that Kate and William's one-year-old son Prince Louis was also in attendance.
On Instagram, the pair said they look forward to competing again next year.