Police were called to Ullswater in Cumbria, near Howtown Outdoor Centre, at around 5pm on Thursday.
An 18-year-old man was also pulled from the water and taken to hospital. He has since been discharged, Cumbria Police said.
The force said the other man, said to be in his early 20s, was already dead when he was pulled from the lake at around 10.45pm.
The man's family have been informed, police said.
Officers do not believe any other people are unaccounted for.