The fire broke out at about 11:55 p.m. local time while the London Duck Tours boat -- which operates on land and water -- was near the Lambeth Bridge and the Palace of Westminster, Britain's The Telegraph reported.
The sightseers were forced to jump into the water as flames destroyed 33 percent of the boat, Britain's The Mirror reported.
"People are clearly cold, wet and in shock but they were pulled from the water really quickly and that's testament to the work of our firefighters, other emergency rescue teams and others who rushed to the scene to help," said London Fire Brigade group manager Neil Withers.
A woman and a child were treated for suspected smoke inhalation at a nearby hospital, the Mirror reported. Others were treated and released at the scene.