The 25-year-old was found unconscious on the sea floor by his father while they were both taking part in the Liam Nissen Memorial event, held annually in tribute to a Mission Beach teen who died several years ago.
It is believed he may have suffered shallow water blackout.
Cairns police Sen-Sgt Greg Giles said it was understood the man and his father had been free diving at Otter Reef, near Hinchinbrook Island, on Saturday morning when the incident occurred.
He said the father had dropped his spear and returned to the boat to get scuba gear to retrieve it.
A short time later he located his son’s spear with a fish still attached, before finding him on the sea floor.
Emergency services have praised the quick actions of those involved who activated emergency beacons on two vessels to raise the alarm, then commenced CPR which continued until they met paramedics and the Rescue 510 emergency helicopter at South Mission Beach.
Queensland Ambulance Service senior operations supervisor Kev Morgan said the man’s pulse returned after being shocked three times and given adrenalin by paramedics, but did not regain consciousness.
