The 16-year-old who has inspired a whole generation of young activists, refuses to fly due to the carbon emissions produced by planes.
"Instead of speaking to me and to the school-striking children and teenagers they should be talking to actual scientists and experts in this area", she said.
The 60-foot (18-meter) yacht is skippered by Pierre Casiraghi, vice president of the Monaco Yacht Club and a member of the principality's ruling family, and German round-the-world sailor Boris Herrmann.
Helen Salmon said "there is clearly something wrong with Hartley-Brewer" after her comments.
The 16-year-old activist, who shot to global fame last year after she started missing school every Friday to demonstrate outside the Swedish parliament, is bound for NY, where she will take part in a United Nations climate summit. She said she did not yet know how she would come home at the end of her travels.
"This is a man who funded Brexit lies, lives a life of excess and chooses to wish an accident on Greta Thunberg".
She said her adventure would have challenges including seasickness but said many people in the world were suffering a lot more than that.
"I will just have to see, get on the boat and see what happens, and that is also very exciting".
"We create an worldwide opinion and movement so that people stand together and put pressure on the people in power".
Watch: Greta Thunberg says she won't meet with Donald Trump.
The toilet is a blue plastic bucket, complete with a biodegradable bag that can be thrown overboard, and meals will be freeze-dried packets of vegan food mixed with water heated on a tiny gas stove.
The youngster is crossing the Atlantic in the 60ft sailing yacht, which is fitted with solar panels and underwater turbines to generate zero-carbon electricity on board.
"You can't really ask for that much if you get to sail across the Atlantic for free", she told AFP ahead of the journey.