Police arrested a 33-year-old man on suspicion of attempted murder after an attack on a 21-year-old woman in Waimänalo.
The man and woman were arguing at 11:22 p.m. Saturday when he struck her, then dragged her into the ocean and held her head under water until bystanders intervened, Honolulu police said.
The man was arrested and being held on suspicion of attempted murder, kidnapping and other charges.
The woman was treated and released.