City lifeguards rescued a 55-year-old female visitor from South Korea who was snorkeling alone Wednesday at Hanauma Bay.
Lifeguards pulled the woman out of the water at 1:40 p.m. from about 10 feet offshore.
Department of Emergency Services spokeswoman Shayne Enright said the woman was unresponsive, without a pulse, when lifeguards brought her to shore.
Lifeguards performed cardiopulmonary resuscitation, and the woman regained consciousness and began breathing on her own, Enright said in an email.
Honolulu Fire Department personnel were called by Ocean Safety to assist, said Capt. David Jenkins.
Firefighters took over care until Emergency Medical Services personnel arrived.
Paramedics transported the woman to the hospital in serious condition.