Eyewitnesses have reported seeing a large number of people swimming in the Mersey as the tour boat sank in Salthouse Dock, part of the Albert Dock complex.
People could be seen throwing life-rings into the water to help those trying to escape.
More than 30 people were rescued and 27 people were taken to hospital, mostly for shock, but all were well enough to be discharged, North West Ambulance Service have said.
An investigation has been launched after 31 people were rescued or had to swim to safety when an amphibious tour bus sank in Liverpool.
Several tourists have been airlifted to hospital, after an amphibious tour bus sank in Liverpool's Albert Dock.