The Lancaster County Sheriff's Office said boaters pulled 61-year-old James Lee from Branched Oak Lake Sunday afternoon.
The 911 call came in at 12:37 p.m. Sunday afternoon.
Sgt. Phil Lang said boaters at the lake saw the man swimming and a short time later he was not.
According to the Nebraska Game and Parks, Lee was not wearing a life jacket when he swam from the shore at Area 14 to retrieve his personal watercraft, which had drifted off the beach.
Boater Daniel Nordgren asked if he needed help and Lee declined. Not long after, he looked back to see if Lee had made it and saw him face down in the water.
He was found face-down in the water about 50 yards from the shore at approximately 12:30 p.m. Sunday.
"It was scary just to see him face down in the water. We had to react and get out there," Nordgren said. "I didn't really think about it. I knew what we had to do. We had to get him out of the water."
He and his two sons rushed to help Lee. They pulled him out of the water and on to their boat to bring him back to shore.
Nordgren and other beach goers started performing CPR. It continued for at least 25 minutes until emergency crews could get there.
Star Care flew Lee to Bryan West where he's in critical, but stable condition.
The Lancaster County Sheriff's Office said the Nebraska Game & Parks Commission is investigating the incident.
Game and Parks said personal watercraft users should beach their craft and secure it to prevent the craft from drifting away.