The body was reported just after 7:30 a.m. by a passerby, said sheriff's Lt. Jeff Jennings.
Responding deputies found the body of a white man, believed to be in his 50s or 60s, who was wearing jeans and no shirt.
Jenkins said it was not clear how the man ended up in the water but that there were no immediate signs of foul play.
The Contra Costa County coroner's office will investigate the official cause of death.
