Video footage posted on social media showed the boat sailing from Festival Pier and briefly stopping outside the Houses of Parliament before continuing down the river to the US embassy in Vauxhall.
Moris, who was accompanied by Gabriel, one of two sons she had by the Australian journalist while he was holed up in the Ecuadorean embassy in London, told reporters that Assange's was a "political case" as six months after a UK judge refused to extradite him to the United States, "he's still sitting" in Belmarsh prison.
"Julian is innocent and what's happened here is that journalism is on trial," she said.
The WikiLeaks founder, who will turn 50 on Saturday, was arrested in London on April 11, 2019, and sentenced to 50 weeks in prison for jumping bail in 2012, when he took refuge inside the Ecuadorean embassy to avoid extradition to Sweden, where he was facing sexual assault charges that were later dropped.
The whistleblower is wanted by the US Justice Department on espionage and computer fraud charges, after WikiLeaks published hundreds of thousands of secret files and classified information that shed light on war crimes committed by US troops in Iraq and Afghanistan. He faces up to 175 years in solitary confinement inside a top security American prison if convicted in the US.
In January, UK District Judge Vanessa Baraitser ruled not to extradite Assange to the United States, citing health reasons and the risk of suicide in the US prison system, but decided that he must wait in prison for the outcome of an appeal filed by US prosecutors.
As part of the events to mark Assange’s 50th birthday, supporters are calling people to join them for a picnic in Parliament Square on Saturday.