Ms Fenton also recalled how Princess Margaret and Queen Elizabeth II were also avid swimmers.
"I can remember seeing when the Queen was young and her sister Princess Margaret - we used to see them swimming in pictures," she said, noting how Prince William told her the Royal Family were all able to swim.
In 1939, the then Princess Elizabeth and Princess Margaret participated in a swimming competition. At age 13, Her Majesty won a Life Saving Medal from the Royal Life Saving Society.
In February this year, the Duchess of Cambridge was seen swimming with Princess Charlotte and Prince George at a luxury hotel complex in Norfolk near their country home, Anmer Hall, according to the Daily Mail. A source noted the children were swimming unaided and even attended a race organised by their mother.
"Both the children swam unaided and were super confident," the source said. "They went under the quite forceful jets in the water."
"Kate politely asked us if it was ok for them to switch it on," the witness continued. "They were doing little jumps off the side into her arms."
"Kate did not move from their sides...George did ask to go outside to use the hot tub but she said 'No'."
Princess Charlotte - who started school at Thomas Battersea's this year - was also seen doing "little duck dives off the side of the pool" and was heard saying: "Look mummy I’m diving in."
However, it's not the only physical activity Prince George and Princess Charlotte enjoy. At the Diana Awards at Kensington Palace, Prince William revealed his children were fiercely competitive when it came to football.
"So when I was chatting with Prince William he said to me, when he plays football with George and Charlotte, when Charlotte’s in goal George says to her that 'Charlotte I’m better than you,'" Olivia Hancock, who was rewarded for promoting gender equality in football, said, according to Express.
The Duke also told her he set the record straight with his eldest son, telling him: "'George, Charlotte could be as good as you.'"
As both William and Kate are advocates of an active and outdoorsy lifestyle, it's no surprise their children have a love of physical activity.