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."
More top news
The Met Office has issued a yellow weather warning for the risk of snow and ice in parts of the Anglia region on Sunday night into Monday.
Touts for unlicensed punt operators could soon be banned from parts of Cambridge.
A few wintry showers are possible during the evening, but most places clear with frost developing.