Four people were taken to hospital after carbon monoxide levels high enough to kill were detected on their Broads cruiser.
Emergency services were called just after 7pm on Saturday after all six passengers on the 21ft boat began to feel unwell simultaneously.
The boat users initially thought that they might have suffered food poisoning after sharing soup on board.
All six passengers on the boat began to feel unwell simultaneously and carbon monoxide levels were high enough to kill.
But firefighters with testing equipment found that carbon monoxide had reached dangerous levels on the boat at Somerleyton marina near Lowestoft, Suffolk.
Four adults who were said to be close to passing out were taken by ambulance to the James Page University Hospital, Gorleston, Norfolk.
The other two adults decided they were well enough not to go to hospital. It is believed that they inhaled deadly fumes from the engine of the privately-owned cruiser which had wafted into its cabin space.
The Government’s Marine Accident Investigation Branch has begun an investigation into the incident.
Firefighters and an ambulance rushed to the scene after carbon monoxide levels posing a fatal risk were detected.
David Burwood, a rescue officer with HM Coastguard Lowestoft and Southwold, said: ‘The symptoms of carbon monoxide poisoning are very similar to food poisoning. You feel sick, light-headed and dizzy.
‘The people on the boat initially thought they had food poisoning from soup they had eaten earlier, but thought it was strange to all have it at the same time.’
Mr Burwood added: ‘At first carbon monoxide levels were recorded at 25 parts per million, which is higher than normal.
‘This then rose to 240 parts per million. To be blunt that is high enough to kill somebody. Four people were taken off to hospital while the other two felt okay to stay with us. And once the area was deemed safe we were stood down.’
Mr Burwood added: ‘The message from us and the fire service is very much the same.
‘Make sure you have a carbon monoxide alarm on your boat, if not you are putting your life at risk it is the silent killer.
‘They did the right thing and called for help when they knew they were in trouble.’
Retired carpenter Alan Frost, 64, and his partner Tina Wilkins, 51, and their dog were killed by carbon monoxide fumes from their new boat in June 2016 at Wroxham on the Norfolk Broads.