Friends of a teen boy who died in a jet ski crash have visited the Sydney river to lay flowers, as his friend who lost an arm remains in hospital.

Mitchell Irvine, 15, was found dead in the water when emergency crews arrived at the Georges River, Sylvania, about 6pm on Tuesday.

His friend, Noah Watkins, 14, was rescued, but his arm was severed in the accident.

Friends and family of the 15-year-old gathered on Wednesday night near the site of the crash, where memorials were laid out.

A wooden cross at the site read “Long Live Mitch”.

Mitchell Irvine has been remembered for his ‘pure heart’. Photo: Supplied

Superintendent Donald Faulds said the cause of the crash remained unknown.

“We believe they were on a jet ski that was owned by one of the families (but) we don’t know what they’ve collided with,” he said.

“(We) are looking at all different areas of the investigation, whether it was something to do with the bridge, something that was maybe submerged, or another item nearby.”

Most debris had been removed from the water but the main body of one jet ski had yet to be recovered, Faulds said.

Both teenagers were wearing life jackets at the time of the crash.

Paramedic Scott McNamara said a police officer had approached him as he suited up for the rescue and described himself as a good swimmer.

The police officer and paramedic swam into the river to save Noah as he slipped in and out of consciousness.

“The victim was floating away from us at the time, it was probably 60 metres we had to swim to catch up to the victim,” McNamara said.

“It’s an absolute tragedy and my condolences to the family of the deceased, but to have saved that young guy, that’s why we do this job.”

Flowers and tributes near the scene of the fatal jet ski accident. Photo: AAP

Friends have laid flowers and crosses in a makeshift memorial, with the words LLM written with pebbles — meaning Long Live Mitch.

Notes left by mourners read: “I’m going to miss you so much bro.

“Take care up there lad, 4eva 15.

“Won’t be the same without you here.”

Mitch’s friend Alana told The Daily Telegraph that he “had such a pure heart”.

“He always decided to help people out, even if he wasn’t good mates with them,” she said.

Alana said all of Noah Watkins’ friends “wish him a speedy recovery”.

“Everyone’s worried about him,” she told the newspaper.

-with AAP