Four men are critically ill after falling from a ship into the water during an exercise drill at the Royal Portbury Dock near Bristol.
An ambulance service spokesman said one man went into cardiac arrest and the others were suffering "extreme hypothermia".
The men, all ship crew members, were taken to Bristol Royal Infirmary after the incident on Monday afternoon.
The men are estimated to be aged between their late teens to early-30s.
Great Western Ambulance Service (GWAS) received a 999 call from the port operators just after 1600 GMT.
A GWAS spokesman said paramedics worked with Avon Fire and Rescue Service to get the men out of the water.
The Royal Portbury Dock is owned by the Bristol Port Company.
In a statement, the company said the four were members of crew from a car transporter ship called the Tombarra, which was in dock.
The spokesman said the accident had happened during a routine exercise drill.
He said that a joint investigation into what happened has been started by the Maritime and Coastguard Agency, the Marine Accident Investigation Branch and Avon and Somerset Police.
Paramedic Danny Hopkins said: "All four patients were taken to the Bristol Royal Infirmary, each accompanied by a critical care paramedic - one of them travelling in Royal Portbury Dock's own ambulance."
He added: "While we don't know for certain how long they were in the water, they have all suffered profound hypothermia - one patient was in cardiac arrest at the scene, while the others are all at risk of doing so as they are gradually warmed up in hospital.
"The next 24 to 48 hours are critical."