A romantic marriage proposal on the US West Coast turned deadly for the bride-to-be when a wave swept her out to sea.

Scott Napper had taken 22-year-old Leafil Alforque to Proposal Rock near Neskowin Beach to pop the question at a place that got its name from couples ready to marry.

Mr Napper and Ms Alforque had been dating since they met on the internet in 2005.

But Ms Alforque had arrived in Oregon on a visa from the Philippines just three days before the fateful trip to the coast, The Oregonian reported.

Mr Napper said the tide had receded around Proposal Rock when the couple began to walk to it. He planned to propose and give her the ring he carried in his pocket.

About 10 feet from the rock, a wave about 3 feet high suddenly came toward them.

"I turned into it to keep from getting pulled under it," Mr Napper said.

By the time he turned to find Ms Alforque, only 4-foot-11 and 93 pounds, she had been caught by the receding waters.

"She was about 30 feet away, getting swept away," Mr Napper said.

The 45-year-old tore off his jacket to get rid of any extra weight, and when he looked up again she was gone.

"That's the last I saw of her," he said in an interview, breaking into tears.