A man who jumped into a dock for a £100 bet was "gambling with his life", rescuers have said.
The Saturday night reveller was dared to take a dip in the water surrounding the Princes Quay shopping centre in Hull.
But Humberside Fire and Rescue Service (HFRS) were called when he found himself trapped within the high-walled waterway.
Firefighters said various hazards made the quay a "potential death trap".
They said "high walls, no ladders and hidden dangers under the surface, combined with a very low water temperature" were among reasons to avoid it.
A HFRS spokesperson warned "every second counts" for those who enter the water, as they would not keep themselves afloat if they started to panic or get tired.
"Cold water shock can kill within minutes," he added.