A spokesman for Norfolk Police said:
"The man was retrieved from the water and was taken to Addenbrooke's Hospital by air ambulance for treatment. He is currently described as being in a critical condition.
"Police and the Health and Safety Executive are carrying out a joint investigation into the incident."
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Very warm with sunshine. Chance of thundery showers late morning.
A hot, humid and sunny start before cloud increases as a narrow band of thunderstorms spreads northeast. These clear during the afternoon.
A fine and sunny end to Friday will then be followed by a warm and muggy night. Warm and sunny on Saturday with the risk of a thunderstorm.