Semaj Johnson and three friends had been swimming for about an hour Wednesday in a pond in the Telfair subdivision on Lady's Island when they decided to swim to the other side.
Johnson never made it, authorities say.
Johnson, 17, of St. Helena Island was pronounced dead at 6 p.m., shortly after his body was found near the bottom of the pond by members of the Beaufort County Sheriff's Office dive team, said Beaufort County Deputy Coroner David Ott.
Johnson would have been a senior this fall at Beaufort High School, where he ran track, according to authorities.
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Witnesses said the four teens were swimming across the pond at about 3:30 p.m. in the Ardmore Avenue area when Johnson began lagging behind, struggled, and went under.
"They had been swimming a while and decided to swim across and (Johnson) decided he wasn't going to make it," Ott said. "He tried to turn back but never made it to shore."
One of the four swimmers lived in a house bordering the pond, which is about 150 yards across and more than 15 feet deep in some places, said Lee Levesque, spokesman for the Lady's Island-St. Helena Fire District.
Members of the fire district combed the pond for nearly two hours until the search was temporarily delayed by sediment stirred from the bottom that reduced visibility in the already murky pond, Levesque said.
The Sheriff's Office's dive team resumed the search at about 5:45 p.m. and found Johnson's body about 15 minutes later.
Firefighters and deputies waded into the pond and held up a large blue tarp to shield the body from onlookers and nearby residents who gathered on the banks of the pond to watch the search effort.
Johnson's body was taken Wednesday to the Medical University of South Carolina in Charleston for an autopsy, Ott said.
Drowning is believed to be the cause of death, Ott said.