Shouts of “Harry, Harry” could be heard, while Meghan posed for selfies with members of the public who stood in the sun to catch a glimpse of the couple on their short visit to Dusseldorf.
The duchess wore a cream halter-neck knitted vest by LA-based designer Anine Bing, and beige wide-legged belted trousers.
The couple’s outing on Tuesday comes after they made their first public appearance in the UK since returning for the Queen’s Platinum Jubilee earlier this year.
The MS Rhein Galaxie which Harry and Meghan were expected to be on, set sail down the River Rhine on Tuesday afternoon shortly before 3.30pm.
A small German police boat travelled slightly ahead of the vessel.
A number of people, understood to be Invictus competitors and their family and friends, stood on the top deck of the large ship, which had the official flags of the games flying upon it.
The couple were understood to be taking a brief sightseeing tour on the boat before heading to the Merkur Spiel-Arena for a press conference at which Harry will deliver remarks.
Delivering a speech at the Town Hall reception, Harry, wearing a suit and white shirt, said he is “filled with such excitement” at the thought of the next Invictus Games having the backdrop of the River Rhine.
He said: “I can’t wait to hear the roar of the crowds, to feel the adrenalin of the competition and join in on the laughter and even the tears, happy ones of course, while we get to witness lives being changed right before our eyes.”
He said he would encourage people to “really lean in” to the games.
He added: “In our most vulnerable moments, encouragement is essential.
“So, I ask that we all do our part to provide space for these incredible servicemen and women to be honest, courageous and true to themselves.”
Showing his trademark sense of humour, the duke opened his speech with a few words of German and wiped his brow to applause as he smiled in relief, adding: “That’s about where it stops.”
He also jokingly asked the people at the small reception whether the beer in Dusseldorf was better than in Cologne, to clapping and nods of yes from those in attendance.
Marking the official One Year To Go event, he said: “It is my absolute honour, along with my wife, to invite the people of Germany to officially start the countdown to next September.”
He encouraged everyone to “lead with our hearts and, most of all, enjoy the ride ahead”.
Harry and Meghan were treated to a red carpet arrival at the Town Hall reception, stepping out hand-in-hand to cheering crowds.
Inside, the pair signed a special guestbook known as the Golden Book, which is also believed to have been signed by the Queen during a visit to Dusseldorf in the 1960s, according to a city official.
During his speech at the reception, the city’s mayor Stephan Keller, recalled the Queen and the late Duke of Edinburgh’s previous visits to Dusseldorf and said they signified the strong relationship between Dusseldorf and Britain since the end of the Second World War.
Harry described those who have worn a uniform to serve their countries, “fighting for their lives, our lives, our freedoms and our democracies”, as “my role models”, as he addressed press at the arena later in the afternoon.
He described Dusseldorf as a “beautiful city” with “a vibrant atmosphere and filled with amazing people”.
“From what I’ve witnessed today already, I have a good sense of how we will bring the motto of these games, A Home For Respect, to life,” he said.
“It is my belief that showing respect and appreciation is just as important as experiencing respect and appreciation. That’s the spirit we’re bringing to Dusseldorf.”
Tuesday’s trip comes a day after Meghan gave the keynote address in Manchester on Monday for the One Young World summit.
The former actress told the 2,000 delegates how her life had changed when she last joined the summit in London in 2019, singling out her role as a wife and a mother, but making no mention of becoming a senior royal the year before.
Harry and Meghan are due to return to the UK for the WellChild Awards in London, where Harry will deliver a speech on Thursday.