Around 7 p.m., the boat was taking part in a holiday boat parade when someone aboard fell unconscious and others became ill, said Vito Maggiolo, a fire department spokesman.
A total of 16 people were on the boat when the incident occurred. Of those taken to hospitals, four were adults and three were children, Maggiolo said.
The unconscious person had regained consciousness, he said.
The cause of the presumed leak was not immediately known.
Carbon monoxide, a colorless, odorless and potentially fatal gas, can be generated in the combustion of carbon-based fuels by engines and other devices, and it can accumulate in closed spaces.
The channel lies between the Hains Point/East Potomac Park area and the Southwest Waterfront.
In some accounts eight people are reported to have gone to hospitals. However, rescue officials said one of the eight was an adult who accompanied a child. The adult did not require treatment, they said.