The US Coast Guard ended a search on Monday for a teenager whose parents were killed after they plunged into the cold, powerful surf in Northern California in a nightmarish chain of events that started when their son tried to save the family dog from drowning.

Mary Elena Scott, 57, and Howard Gregory Kuljian, 54, both drowned during the incident on Saturday, said Ariel Gruenthal, a deputy coroner.

Flowers rest on a large log meters from the breaking surf of the Big Lagoon beach near Trinidad, California, on Monday. A family that tried to rescue their dog from a powerful surf at the beach in Northern California was swept out to sea, leaving the parents dead and their 16-year-old son missing, authorities said. Jose Quezada / The Times-Standard via Associated Press

The boy, Gregory James Kuljian, has not been found and is presumed dead.

Powerful, three-meter waves had pulled the dog into the ocean on Saturday as it ran to retrieve a stick at Big Lagoon, about 500 km north of San Francisco, authorities said.

The 16-year-old boy went after the dog, prompting his father to attempt a rescue, said Dana Jones, a state parks district superintendent. The teenager was able to get out of the waves but then went back into the water with his mother in search of his father.

"Both were dragged into the ocean," Jones said, adding the dog got out of the water on its own.

The couple's daughter and the boy's girlfriend watched the tragedy unfold. The daughter called police, but by the time help arrived, it was too late.

Jones said a park ranger had to run one km to get to the beach because his car wasn't made to handle the rugged terrain. When he arrived, he wasn't able to get to the family members because of the high surf, she said.

Coast Guard Lieutenant Bernie Garrigan said the search for the teenager was stopped because a person without a wetsuit could not survive for long in the surf due to frigid waters.

Saturday was overcast and a bit damp, and the winds were light at Big Lagoon beach, a steep shoreline where the waves roll in and crash onto the sand, making the area dangerous, officials said.

Signs are posted near the beach parking lot warning beachgoers not to turn their back to the surf, and to pay special attention to "sneaker waves", or swells that can seemingly appear from nowhere and violently smash onto the beach, Garrigan said.

"Because the beach is designed that way, when that three-meter wall breaks, it surges up on the beach and surges back really fast," he said. "It's like a cyclical washing machine."

Rescuers eventually retrieved Scott's body, and Howard Kuljian's body washed ashore.

The Coast Guard deployed a helicopter and two motor life boats to search for the teenager, but thick coastal fog made the search difficult.

The Associated Press