Harry Lee, 51, went missing at about 2:30 p.m. Saturday, when his fishing boat was found adrift in the Delta. The keys were still in the boat's ignition and the engine was off, according to the U.S. Coast Guard.
Officers also found Lee's vehicle at the Lauritzen Yacht Harbor at 115 Lauritzen Lane. He was nowhere in sight, according to the sheriff's office.
A dive team from the sheriff's office searched for Lee near the location of his boat on Wednesday, but still haven't located him.
"We were still searching intensely the last few days, but we'll now revert to normal patrol," sheriff's spokesman Jimmy Lee said.
The Coast Guard suspended its search for Lee on Saturday. The search spanned approximately 46 nautical miles, the Coast Guard said.
Lee is thought to have been wearing a manual lifejacket that must be inflated using a pull string.
A Coast Guard official said Saturday that it is unlikely that Lee inflated his lifejacket.
Lee has been entered into a missing persons database and local hospitals have been told of his disappearance.
He is described as an Asian man who is 5 feet 8 inches tall, weighs 135 pounds, and has gray hair and brown eyes. He was last seen wearing an olive-green sweatshirt and dark pants.
Anyone with information about Lee's disappearance is asked to call the Contra Costa County Sheriff's Office at (925) 646-2441.