A problem with the foam used to provide buoyancy was at the centre of both recent "Duck" amphibious tourist vehicle accidents, according to an official report.
The incidents - one in Liverpool in June and one on the River Thames in London in September - resulted in passengers, including small children, going into the water.
"It was extremely fortunate that, on both occasions, there were no serious injuries or loss of life," said the Marine Accident Investigation Branch (MAIB) which reported both accidents in one report today.
The MAIB is now recommending that the vessels not be permitted to operate until the foam problem was sorted out.
Chief Marine Accident Inspector Captain Steve Clinch said: "The sinking of Wacker Quacker 1 in Salthouse Dock, Liverpool, and the fire on board Cleopatra on the River Thames were serious marine accidents involving WW2 DUKW amphibious vehicles that had been modified for use as sightseeing tour vehicles.
"Both accidents resulted in the rapid abandonment into the water of passengers, including small children, and crew. It was extremely fortunate that, on both occasions, there were no serious injuries or loss of life."
He went on: "Although one accident involved the sinking of a DUKW as a result of flooding, and the other involved a fire, the link between both events is the foam inserted into the DUKWs to provide buoyancy.
"In the case of the Liverpool sinking, insufficient foam had been inserted into the DUKW; the accident on the Thames was caused by the foam being packed too tightly around some of the DUKW's machinery, resulting in friction and overheating that caused the fire."
Capt Clinch went on: "Attempts to resolve how much foam is required in the DUKWs, and how it should be inserted, have been ongoing for several months, during which time the vessels have continued to carry passengers.
"However, the results of the investigation into the fire on Cleopatra indicate that the current method of inserting foam is not working as it compromises the safe operation of the vessels.
"I have therefore recommended to the Maritime and Coastguard Agency that the DUKWs should not be permitted to operate until the required standards of buoyancy and stability can be achieved without adversely impacting on their safe operation. "
The Liverpool incident on June 15 resulted in 31 passengers and two crewmen abandoning the vessel and going into the water. All were recovered without serious injury.
The Thames incident on September 29 involved the vessel's master beaching the vehicle before ordering the evacuation of passengers and crew. There were no serious injuries.
No one was available for comment from either Liverpool's Yellow Duckmarine company or from London Duck Tours.