Jack Susianta, 17, died after jumping in a canal at Walthamstow Marshes in July 2015.
He had suffered a psychotic episode and left his home in Hackney, east London.
PC Tom Griffiths said he saw the teenager go underwater and believed it was a "deliberate act" to evade officers.
"I think it's just a sad, unfortunate truth that we were not able to save him," he said.
His colleague PC Richard Hughes said he believed the teenager might have "fought us off in the water".
The officers were giving evidence before an inquest jury at St Pancras Coroner's Court.
Mr Griffiths continued: "I thought maybe he was going to swim down river. I saw him in the water, he was treading water and he did go under a couple of times.
"To me it looked like he was going down to swim underwater. It looked like a deliberate act.
"He did that a couple of times but he did not look like he was in difficulty at any point."
Jurors heard how it took nine minutes between Jack jumping into the canal which forms part of the River Lea and an officer, PC Richard Wilson, requesting permission to go after him.
This was about four minutes after the teenager was last seen.
Another officer involved with the search, PC Anthony Daly, said he believed it was already too late.
"The length of time Jack had been submerged, he could not have survived. Several minutes, that's too long for somebody to hold their breath," he said.
He said it was too dangerous for him to go in the water.
"As a person seeing someone in that position, who clearly needs help, it's horrible. Your natural instinct, you want to help.
"I don't believe me entering the water to try and help him would have been productive at all."
Asked if all officers should be required to swim, Mr Griffiths said the unit rarely dealt with incidents in the water and all police officers would have to be trained to make it an effective policy.
He added that officers did look at other ways to save Jack.
The inquest continues.
A teenager who jumped into a canal while fleeing police did not look to be in difficulty and refused help before he drowned, an inquest heard.