A 33-year-old Canadian man was found unresponsive just before 5 p.m. Monday at Hanauma Bay.
Ocean Safety lifeguards rescued the man, who had been snorkeling, about 20 feet offshore. They brought the man to shore and performed CPR.
Emergency Medical Services personnel responded and continued with advanced life-saving treatment and transported him in critical condition to a hospital.
Honolulu Emergency Services Department spokeswoman Shayne Enright said the man is said to be visiting from Canada.