Steven Mitchell died last July after getting into difficulties in the River Roe in Limavady when he waded out into deeper waters to retrieve a football.
His grandfather Joseph Milligan told an inquest hearing in Belfast on Tuesday that the 15-year-old was not a good swimmer and that he had panicked when he realised he was out of his depth.
Steven, from Doagh in Co Antrim, was the eldest of six grandchildren who had accompanied Mr Milligan, his son and two daughters to Roe Valley Country Park on a hot summer day.
All the children went into the river to play and Steven, who was wearing a T-shirt, shorts and trainers, waded out the furthest.
"He just went out too far and he couldn't find the bottom and he panicked," Mr Milligan told coroner Jim Kitson.
The schoolboy's uncle, Joseph Milligan Jnr, was the first to swim out to try to rescue him - but, when he reached the spot where the teenager had disappeared, he could not find him.
He then got into difficulties himself and was forced to return to the riverbank.
Steven's grandfather, who has a weakness in one arm due to arthritis, then entered the water in a bid to reach Steven.
"I did get hold of his arm at one point, but I wasn't strong enough to hold him up with one arm," he said.
Steven was driving me down as well, as my lungs were filling with water.
Joseph Milligan Snr, Steven's grandfather
Mr Milligan Snr later told the court: "I just couldn't keep him up - he slipped away from me."
A passer-by, David Seed, then went into the river and found the schoolboy below the surface.
"I grabbed hold of the boy and dragged him to shore," he said.
Steven's uncle had returned to the water by this stage and helped Mr Seed pull his nephew to the riverbank.
Mr Milligan Jnr told Mr Kitson that, despite the warm weather, the water was very cold.
"I hit it running and it took the breath out of me," he said.
The teenager's uncle broke down as the coroner praised him for doing his best to save his nephew.
On shore, Mr Seed started resuscitation efforts. He said he tried for around 15 minutes to revive him before members of Steven's family took over for another 20 minutes.
Paramedics took around 30 minutes to reach the unconscious teenager after receiving the emergency call. A lengthy drive to the park was compounded by the inaccessibility of the site.
Emergency medical technician Manus McCaul told the court that, having taken 15 minutes to drive to the park, they arrived to be informed that Steven was 15 minutes walk from the car park.
He said he and his colleague tried to drive to the scene, but within 500 yards they reached a fence that prevented them going any further.
Mr McCaul told the court they to make the rest of the journey on foot, carrying resuscitation equipment.
"It was a very hot day and we were carrying all our stuff and it did take us 10 to 12 minutes to get there," he said.
When they arrived, the ambulance crew took over resuscitation attempts and delivered a number of electric shocks to try to restart Steven's heart.
They were soon joined by senior paramedic John McClintock.
I just wish I could have done more.
Joseph Milligan Jnr, Steven’s uncle
Two helicopters were scrambled - one from the PSNI and the other from the Irish Coastguard in Sligo.
Only the smaller police helicopter was able to land at the site and it took Steven to Altnagelvin Hospital in Londonderry.
Mr McClintock said a weak pulse was detected just before Steven was put in the helicopter.
"Unfortunately, as we lifted off the field, the pulse ceased," he told the court.
Resuscitation continued at Altnagelvin, but efforts to save the teenager were unsuccessful.
Officials from Roe Valley Park told the inquest that health and safety officers from the Northern Ireland Environment Agency (NIEA) were still considering whether the tragedy should prompt the erection of additional warning signs.
But they pointed out that signs warning of deep water and a fast flowing river were already within the park and that health and safety measures complied with current standards.
Confirming the cause of death as drowning, Mr Kitson commended the efforts of those who tried to save the boy.
"This is a quite horrific tragedy," he said, expressing his condolences to the family.
He said that, while no one involved in the fatal incident could be criticised, the case did serve as a cautionary reminder of the dangers of swimming in rivers.