Neither George – who turned three on Friday – nor his younger sister, Princess Charlotte, 1, attended the Sunday event, which included having William and Kate tour the headquarters of Ben Ainslie Racing (BAR) to meet the team plotting to bring the 'Auld Mug' back to Britain for the first time since the yacht "America" won the inaugural trophy way back in 1851.
"She was lovely beautiful and friendly," Tiyah Windslade, 17, told reporters shortly after presenting a bouquet to Kate on behalf of the 1851 Trust, which is using the America's Cup to inspire a new generation to become involved with the marine world – with a strong focus on those from underprivileged families.
"I almost started crying!," Tiyah added about meeting with Kate, who paired a BAR team shirt bearing the words "The Duchess of Cambridge," with tight skinny jeans and beige espadrille wedges.
"She spoke a lot about mental health and how we need to stop people from feeling ashamed about themselves and come out and be very open about their mental health. And she sad that we are the voice reaching out to people because we are working with them on the ground.
"It is great to have them involved," Tiyah continued. "They give us so much support."
"I was chosen to present the bouquet," she added. "They told me a couple of moments before and it was so sudden, I was like 'I didn't know I was doing that!?'
"I just knew I was meeting them and they said, 'Are you OK to take the lead and give them the bouquet?' So I sad, 'Er, yeah, OK! I'm already so nervous, now you've really shaken me!' "
Thankfully, the royal couple calmed her nerves immediately.
"He was really lovely too", Tiyah added about William.
Following an hour-long tour of the British team's headquarters, William and Kate boarded a BAR catamaran flying a royal standard to sail around Portsmouth. They then took a seat in the bleachers among thousands of sailing fans to watch the high-speed racing action, which drew teams from all across the globe.
The couple will later visit the Race Village to meet the six international crews and hand out the trophy designed by competition winner Kay.
This is Kate's third visit to the Portsmouth leg of the America's Cup World Series. In May, she had an exhilarating trip in the water, sailing around the Solent with four-time Olympic champion – and reigning America's Cup winner – Ainslie and his crew. Ainslie told PEOPLE after the event that it was "a wonderful sail."
A few days later, Ainslie posted exciting GoPro-style footage of the trip, showing Kate in full control alongside her colleagues on board.
Kate and William are both experienced sailors and frequently took a day on the seas close to their first marital home in Anglesey, north Wales. Kate also beat William in a sailing race in Aukland, New Zealand, during their tour of the country in April 2014.
During their first royal tour in the summer of 2011, the famously competitive couple also took charge of a dragon boat race across a lake in Canada.