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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Outbreaks of rain at first today, turning brighter and drier into the afternoon. Breezy but milder. Highs of 13 deg C (55 deg F).