A 27-year-old Blackburn man was pulled from Derwent Water in Cumbria after he stopped breathing and lay unconscious on the lake bed for 10 minutes.
The man got into difficulties after he tried to retrieve a hire boat and was pulled from the lake late Saturday evening before beinf taken to hospital.
On Sunday the man, who has not been named, was in a "serious condition" at the Cumberland Infirmary in Carlisle.
He had dived into the lake when he saw the boat drifting away, after he and a group of friends had hired it during a camping trip.
Inspector Paul Kissack, of Cumbria Police, said: "This man was underwater for in excess of five to 10 minutes.
"Two police officers, who had gone out on to the lake with a local rescue boat, dived in and brought him back to the surface.
"He was clearly unconscious and had stopped breathing."
Mr Kissack said the officers, who had used powerful torches to locate the man, were "lucky" to have managed to find him in time.
"By the time he was brought back to the shore, ambulance crews were at the scene, and worked on him extensively to bring him round," he said.
"He was initially taken to a cottage hospital in Keswick, before being transferred to the Cumberland Infirmary."
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