Jordon Lyon disappeared under the murky black water of John Pit in Standish Lower Ground as he struggled to keep eight-year-old stepsister Bethany's head above water.
Moments earlier the children had been happily playing and hunting for tadpoles at the water's edge.
At an inquest into Jordon's death, deputy coroner Alan Walsh said it was the most tragic case he had ever dealt with.
The Bolton hearing was told two fishermen waded into the water and helped pull Bethany to safety, but could not reach Jordon.
The three found the boy, by then unconscious, and got him to the side. Sgt Lippitt began trying to resuscitate him.
The inquest heard Jordon was underwater between 10 and 30 minutes. An air ambulance arrived and paramedics continued trying to resuscitate him