Charlotte was snapped sticking her tongue out at onlookers, much to the amusement of her mother.
It has now been revealed that she was sticking out her tongue at granddad Michael Middleton who she spotted in the crowd.
The Duke and Duchess of Cambridge were competing against each other all in the name of charity, with the Duke and Duchess of Cambridge skippering their own boats of six celebrity ambassadors.
It was TV personality Bear Grylls and his crew that took home the golden trophy, leaving Kate with a giant wooden spoon for her efforts on the waves.
Although each of the celebrities was representing their own charities, Kate sailed for the Royal Foundation while William battled it out for Child Bereavement UK.