An autopsy revealed Mark Gibbs, the 51-year-old Tustin man who authorities said became lost from his diving group off Shaw's Cove in April, died from drowning.
Investigators are also conducting a toxicology test to determine whether drugs or alcohol were in his system, but those results will not be available for another six weeks, said Daniel Aikin, Orange County supervising deputy coroner.
Gibbs became separated from the group while diving on April 12. Diving classmates did a head count and determined Gibbs was missing.
Rescuers found him unresponsive with an empty scuba tank 25 feet below the ocean's surface at 12:40 p.m., more than an hour after the first 911 calls reported him missing.
Lifeguards placed Gibbs onto a boat and started CPR. He was in cardiac arrest en route to Hoag Hospital, Laguna Beach Marine Safety Chief Kevin Snow said at the time.
Gibbs died at Hoag later that day, according to the Orange County coroner's office.