Not long beforehand, Charlotte made the crowd erupt with laughter when she stuck her tongue out as Kate encouraged her to wave.
William and Kate competed against each other, and a host of celebrities, including comedian John Bishop, on a cloudy but relatively calm day around Cowes.
The event took place a day earlier than planned due to forecast bad weather on Friday.
The dull weather did not stop the couple sporting shorts for the two races but a royal win was not to be, with Grylls winning overall.
William finished tied third and Kate finished last.
Ahead of the first race, the boats were recalled to the starting line after an initial false start in which some of them were over the line.
And in the second race, many boats – including Kate’s – were disqualified for not going back and starting the race at the line.
The duchess, wearing a baseball cap with her hair in a ponytail, sat on the side of her boat alongside crew members for the afternoon of sailing.
The yachts represented causes supported by William and Kate.
William and Kate hope the King’s Cup regatta will become an annual event, increasing awareness of the benefits of sport, while also raising support and funds for their causes.
The event gave the couple the chance to renew their sporting rivalry.
The duke trounced his wife when they competed in a dragon boat race in Canada in 2011, but Kate got her revenge three years later when she won a sailing event in Auckland during their tour of New Zealand.
William took the honours again during a visit to the German city of Heidelberg in 2017 when he was victorious in a riverboat race against his wife.