More than 30 people were rescued or had to swim to safety when an amphibious tour bus sank in Liverpool's Albert Dock.
A number of people were taken to hospital after the Yellow Duckmarine vessel went under just before 4pm on Saturday. A "multi-agency investigation" has been launched into the sinking.
A rescue operation - involving police, ambulance, coastguard and the RAF - was mounted by the emergency services and 31 people were helped out of the water. The Royal Liverpool Hospital said a final total of 18 people were treated in connection with the incident. There were no serious injuries and all patients have been discharged.
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 and added: "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. Then, in May, the Queen and Prince Philip had a ride on one of the yellow Duckmarine buses when they visited the region as part of her Diamond Jubilee tour to celebrate 60 years on the throne.
Writing on Twitter, the Mayor of Liverpool Joe Anderson refused to be drawn on the future of the vessels until he knew everybody involved in the latest incident was safe. He wrote: "Albert Dock Duck incident, look I will not make any official comment on future of these ducks until we know people are all ok+accounted for."
According to the Liverpool Echo, Pearlwild Ltd, which operates the fleet, faces a separate investigation by the North West Traffic Commissioner, with a public inquiry set to be held later this month amid concerns over the operation of the fleet of wartime vehicles.