Jose Barrera, 32, of Manhattan, did not know how to swim, police said, but he apparently went alone and unsupervised into the backyard swimming pool at a friend's home in East Patchogue.
"All we know for certain is he didn't know how to swim," said Homicide Det. Robert Doyle. "His family knew him as a non-swimmer."
Barrera, who was visiting his wife, stepson and others, apparently left the group inside the house and went out to the pool, police said. He was away from them for about 15 minutes when his wife went to look for him.
"His wife actually discovered him at the bottom of the deep end of the pool, which is 8 feet deep," Doyle said.
Barrera was pulled from the pool and 911 was called, Doyle said. Efforts by emergency responders to revive him were unsuccessful. Barrera was pronounced dead at Brookhaven Memorial Hospital Medical Center at 1:52 p.m.
Barrera was wearing cutoff jeans and no shirt, Doyle said. He had no known health problems, police said, and alcohol is not believed to be a factor in his death. Investigators will await a medical examiner's report for a final determination of cause of death.
The in-ground pool is 18 feet by 36 feet and appears to meet municipal requirements, police said.
Barrera may have entered the shallow end and then walked into the deep end, where the water would have been over his head, Doyle said.