Officials said in a news conference that they continue to find debris, but they have not located any survivors.
Search teams scoured a 644-square-mile area.
The crash happened Thursday just after 7 p.m. about 20 miles off the coast of San Clemente Island. The island is used by the Navy for training.
"There were two large explosions, one after the other. It looked like two tanks exploded out there. No sound, just a very large ball of fire came up. Then they dived down again, glowing after that into the horizon," said Ron Newcomb, a witness.
The Coast Guard plane involved in the collision was reportedly conducting a rescue mission, trying to find a person rowing to Catalina Island in a dinghy.
The military chopper involved was an AH-1 Super Cobra, an attack helicopter belonging to Marine aircraft group 39 headquartered at Camp Pendleton. Two pilots were believed to be on board.
Officials said the helicopter was conducting a routine night exercise with three other helicopters when it collided with the Coast Guard plane.
Officials said a person could survive more than 20 hours in the waters. The water conditions are good, clear and calm, with temperatures in the mid 60s.
"Our people are highly trained in survival techniques. They are very highly fit, physically fit. They are able to survive things that you would otherwise be surprised at," said Rear Adm. Joseph Castillo from the U.S. Guard.
"We are continuing to search with the hope of finding survivors."
Though a rescue operation continues, the pentagon says it's unlikely that survivors will be found.