The Contra Costa County Sheriff's Office received a call at about 12:50 a.m. from someone reporting that there was a man in the water near Werner Dredger Cut just north of Discovery Bay. The man was splashing around and yelling for help, sheriff's spokesman Jimmy Lee said.
A short time later, the witness saw the man go under.
The sheriff's office's marine patrol and helicopter responded along with the U.S. Coast Guard and began searching for the man.
Investigators first found a submerged 11-foot fishing boat in the area, but they didn't find a victim.
They continued searching until divers from the sheriff's Volunteer Services Unit found the man's body at about 7:30 a.m.
He was identified as 35-year-old Songkram Sangcharoon, a resident of Antelope, an unincorporated area in Sacramento County. The coroner's office conducted an autopsy on Sangcharoon's body and determined that he drowned.
Deputies continued investigating the incident and learned that there might have been a second man in the boat with Sangcharoon, and returned to the area to search for him around 3 p.m., Lee said.
At about 6:15 p.m., the Coast Guard and sheriff's office marine patrol unit located a second body. Coast Guard Lt. Simone Mausz said the victim appeared to be a man in his 30s.