The Antioch Fire Department has labeled this a recovery mission, indicating they do not believe the missing boater is still alive.
Authorities say they are now conducting a grid search using two sonar devices in murky water that is about 20 feet deep, 300-400 yards from shore.
"It was some sort of mechanical problem with the boat, and the individual was trying to rectify the problem when he fell in," said Dept. Chief Chris Lienhardt, Antioch Fire Department.
The Antioch Fire Department rescue crews put divers in the water just before 6:30 p.m. Authorities say the man was with a number of people, reportedly family members, when he went in.
Friends and acquaintances in this tight-knit boating community came to offer help and support on Tuesday night, and many of them knew the missing man.
He was very athletic, and I'm shocked that this happened to him. He was a very good swimmer, a skier, and a very good athlete and a really nice guy," said Cindy Court, friend of the victim.
The Antioch Fire Department has not yet identified the missing boater, but says he is a white male in his early 40s. His wife and children have been on the scene of the search all night.