Patrick Kim McDermott, 48, left San Pedro, 20 miles south of Los Angeles, June 30 on an overnight fishing trip. The Los Angeles Police Department said McDermott has been listed as missing.
Coast Guard officer Nathan Henise told The Associated Press Monday that McDermott was last seen on the fishing vessel and that his personal items were found on the boat.
"We're treating it as a missing person case," Henise told the AP. "Everyone is being interviewed, everyone on the boat."
Newton-John on Monday urged anyone with information on his disappearance to contact authorities.
"I'm hopeful that my treasured friend is safe and well, and I'm grateful to the officials who are working so hard to find Patrick, whom I love very much," she said in a statement. "I ask anybody with information that could help to please, please come forward."
It was the first public statement by Newton-John about McDermott's disappearance.