More than 30 people had to be rescued after a Yellow Duckmarine tour bus in Liverpool’s Albert Dock sank.
Tourists were rescued or had to swim to safety when the amphibious tour bus went under just before 4pm on Saturday.
A "multi-agency investigation" has been launched into the sinking - the second in three months.
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.
Of those, 18 people 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 understood that 28 people were led to safety from the vessel, 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 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.
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.
A spokesman for the Albert Dock said: "Following the incident, Albert Dock directors would like to praise the response of the emergency services and its onsite security team and will co-operate fully with any investigation."
A spokesman for Yellow Duckmarine said the company was working closely with the regulatory body, The Maritime & Coastguard Agency (MCA), and Merseyside Police.
The spokesman said: "Our team followed their emergency response procedure, ensuring the safe disembarkation of passengers on board."