Isaiah Sanchez, 22, of El Rio was last seen in the water shortly before 8 p.m. on July 11.
He was with two friends when their small boat developed mechanical problems, said sheriff's spokeswoman Kelly Hoover.
Sanchez and another man, whose name has not been released, ended up in the water, while a third man in the boat signaled to people on shore that they needed help.
After 911 was called, county park rangers who responded rescued two of the men, but were unable to find Sanchez.
"For the past several days, the Santa Barbara County Air Support Unit has been conducting periodic flyovers of the area in hopes of finding Sanchez," Hoover said. "Due to zero visibility conditions in the lake, the sheriff’s dive team has not been able to go back in the water and search. The team may get called back in if conditions improve."
Rangers have continued to conduct surface patrols by boat using sonar equipment, and several county lifeguards were brought in Thursday to help with visual searches, Hoover said.
Also aiding in the search have been personnel from California State Parks and the Santa Barbara County Fire Department, who responded with jet skis and are searching the shallow areas of the lake and shoreline, Hoover said.
Members of Sanchez's family also reportedly have been maintaining a vigil at the lake while the search has continued.