Rescue crews started searching for the diver, an off-duty Los Angeles firefighter, just before 10 p.m. Wednesday, when he was reported missing in the water near Pier J.
As of Thursday afternoon they had not located him and authorities said the operation would "transition from a search and rescue mission to a recovery operation."
"As search operations have continued over the last 19 hours, the survivability profile of the diver and decreased visibility have made it difficult to sustain prolonged search operations," the Long Beach Fire Department said.
Four people went out on a boat lobster fishing near the Long Beach port area Wednesday night. Three went into the water for about 45 minutes but only two returned to the boat.
Long Beach first responders were called around 9:45 p.m. to help search for the missing diver. He was last seen in the area near Pier J and 1600 S. Harbor Scenic Way.
His name has not been released.
At least two dozen personnel from multiple agencies were involved in the search, including the LAFD, the Long Beach Fire Department, the U.S. Coast Guard, the Los Angeles County Fire Department, the Los Angeles Port Police Department and the Long Beach Police Department.
With the assistance of sonar and other technology, recovery efforts are expected to continue through Thursday night.
The diver's family members were at the scene, but declined to speak publicly.
"The decision to transition operations to a recovery mission is not taken lightly," the fire department said. "There were many factors that contributed to this decision. As we make this transition, our thoughts and prayers continue for the family of the missing diver and for our brothers and sisters at the Los Angeles City Fire Department.
City News Service contributed to this report.