Long-distance runner Reza Baluchi was picked up off the Florida coast on Sunday and his "hydropod" was being towed back to shore, Coast Guard Petty Officer Mark Barney said.
Mr Barney said the man set off from Pompano Beach on Friday, despite receiving a letter from the Coast Guard on 15 April warning him not to attempt the 3,500-mile journey following the Bermuda triangle and back to Florida via Cuba and Puerto Rico.
Mr Baluchi, a 44-year-old originally from Iran, had been told he was "not authorised to depart" because his vessel - which featured dozens of buoyancy balls, a GPS tracking device and water filter - was "manifestly unsafe".
Captain Austin Gould, Coast Guard Sector Miami Commander, said: "This was an inherently unsafe voyage that put the lives of Mr Baluchi and other mariners in danger."
The Coast Guard added that the voyage was halted because he "violated a USCG order" not to go without a safety boat.
Mr Baluchi could face a fine or have to pay the cost of picking him up, Mr Barney said.
It was the second time he had attempted such a journey.
In October 2014, his GPS device fell into the ocean around 70 miles off St Augustine. The US Coast Guard rescued him from the ocean in an operation which cost more than $140,000.