Gloucestershire Police said rescuers had pulled the body of a man, 20s, from the water at Cotswold Country Park and Beach shortly before 8.50pm last night.
“He was sadly pronounced dead at the scene and the coroner and the man’s next of kin have been informed,” a spokesman added.
Specialist crews from South Western Ambulance Service, Gloucestershire and Dorset and Wiltshire fire services joined the search on Tuesday afternoon, together with a police helicopter and rescue divers.
Police said they had been called at around 1.40pm to reports of a missing swimmer. However, the “swimmer” was in fact a man looking for his missing dog.
Eyewitness Agnes Baron, 37, told the Wilts and Glos Standard: "I saw he was struggling in the water and shouting for help. I ran and raised the alarm."
Two men jumped into the water to help the stricken dog owner: "They got to where he was last seen but could not find him."