A number of people were taken to hospital after the Yellow Duckmarine vessel, right, went under just before 4pm yesterday. A "multi-agency investigation" has been launched.
A rescue operation – involving police, ambulance, coastguard and the RAF – helped 31 people out of the water.
Of those, 17 were taken to The Royal Liverpool Hospital for treatment, mostly for shock, but all were well enough to be discharged.
Nobody was trapped inside the vessel, the fire service said.
The company runs tours on the city's roads with the promise of a "splashdown" ending.
It is the second time in three months that one of the yellow vehicles has sunk.
It is understood that 28 people were led to safety from the bus, including a baby whose mother held her above the water on the roof of the sinking craft. Three more were saved from the water by firefighters.
A spokesman for Merseyside Police said everyone had been accounted for, adding: "A police cordon remains in place at the scene and a multi-agency investigation into the full circumstances of the incident is ongoing."
Eyewitnesses reported seeing a large number of people swimming in the Mersey as the vessel, one of four in the company's fleet, 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.
In March, the entire fleet was ordered out of the water after a bus, which was not carrying passengers, sank.