The helicopter landed shortly before noon in the section of the river near 79th Street by the New York City Marina.
Deputy Fire Chief Thomas McKavanaugh said the helicopter had taken off from the Wall Street Heliport and lost power after 12 minutes in the air. The pilot used the craft's pontoons, and it remained upright as it landed.
"The pilot did a terrific job considering he'd lost his engine power," McKavanaugh said.
The passengers were two adults and two children from Sweden, he said. No one was injured, but the tourists were taken to the hospital for observation, authorities said.
Sebastien Berthelet, visiting from Montreal, was on a boat when the craft went down and went over to help, bringing the pilot back to shore.
He said he complimented the pilot on the landing, but the pilot said it could have been smoother.
"I said, 'Well, it could have been worse, too,'" Berthelet said.
Another boater brought the family back to shore, and they were all calm, he said.
"They all seemed very healthy. They were all shocked, of course," Berthelet said.
The Federal Aviation Administration said the helicopter, a Bell 206, is registered to New York Helicopter, which offers sightseeing tours ranging from $139 per person for a 15-minute flight to $295 per person for a 25-minute flight. A call to the company went unanswered.
The FAA and the National Transportation Safety Board are investigating.
In 2009, a US Airways flight landed safely on the Hudson after striking geese. All 155 people aboard survived.
Associated Press photographer John Minchillo in New York contributed to this report.