Police and other emergency services were called to the park at around 4.30pm yesterday.
"The teenager is believed to have drowned after falling into the river Roe," a PSNI spokeswoman said.
One source suggested that the boy had gone into the river to retrieve a ball when he went underneath the water.
DUP councillor Alan Robinson said the child may have been visiting the park from Co Antrim.
"I'm led to believe that there was a young, 15-year-old male who had travelled down from somewhere up the country, Co Antrim direction, who had been playing in the water," he said. "I'm led to believe there was an incident and he was removed from the water, and I'm told that he subsequently died later in hospital."
Ulster Unionist councillor Jack Rankin said: "My heart goes out to whoever it is.
"It is very sad losing a young fellow of that age.
"With the hot weather, this is the attraction – to rivers. Incidents like this have happened in other places."
Sean McCarry, regional commander for the Community Rescue Service NI, said: "We had temperatures up there of 25 to 26 degrees yesterday, so I imagine being close to the water can be very tempting.
"I know rivers and cool water may look inviting but if you look into the figures across the UK, there's been 12 reported inland water deaths in recent months. That number will be up to 13 today."
Mr McCarry said people were usually aware of the dangers of the sea, but warned that anywhere there is water there is a risk to life, whether in quarries, lakes or streams.
"For every 10 people who are drowned at sea, one is drowned inland. You are never further than a quarter of a mile from water in Northern Ireland, we have to live with it, and be safe with it," he said.
The teenager's death is the fourth drowning of a local since the start of June.
On June 2, 15-year-old Kevin O'Hare and Colin Polland (39) drowned at a disused quarry near Annalong, Co Down.
And a father from Beechgrove Park in south Belfast died saving his 24-year-old son while on holiday in the north-western Spanish region of Galicia in mid-June.
Owen O'Toole (57) and a local man rescued his son, Niall, from the sea after he got into difficulty while swimming. But the strength of the waves overcame the father-of-two and he drowned.
Summer swimming advice: