NT Police has confirmed a man participating in the annual Fannie Bay Swim Classic has died, but says his death is not being treated as suspicious.
Darwin Surf Life Saving Club president Ray Hall was in tears as he spoke about the incident.
"One of our committee members passed away from a heart attack during the swim," he said.
Mr Hall said attempts were made to revive the man, believed to be about 60.
"He was pulled out of the water very quickly by our surf lifesavers and he was getting high-quality CPR immediately in the boat and then brought to shore," he said.
"We're lifesavers, we're very well trained for these sorts of activities.
"He was getting fantastic help immediately.
"Unfortunately, our efforts didn't result in a positive outcome."
More than 300 people took part in the event on Sunday morning, swimming a 2.1km circuit from East Point to the Darwin Trailer Boat Club.
Prior to the race, organiser Stephanie Burton said about 50 safety officers would be on the lookout for dangers such as crocodiles.
"We take the water safety very seriously during the dry season there is less likelihood of having stingers and crocs in the water," she said.
"We have a huge contingent of water safety that this year is the biggest that we've ever had as well."
Ms Burton said there was also a drone and powered vessels on the water to keep an eye on swimmers.
The swim is a Darwin Life Saving Club fundraiser and today marks the club's 16th event.
It's the Top End's only ocean swim.
