Foxfish, it is probably easier to quote direct from Dr Carl Edmonds' advice to defence on this matter rather than answer all your questions. I hope that this will provide the answers:
"Cause of Death, in order of probability.
1. Drowning after salt water aspiration. Contributors include panic, fatigue, negative buoyancy, negative pressure inspiration, swimming against currents, inexperience.
2. Vomiting with aspiration and/or laryngospasm
3. Combinations of 1,2 and 3.
4. Pulmonary barotrauma of ascent with air embolism. Contributions from the use of positive pressure respiration during rescue, and aggravated by resuscitation attempts on the surface
5. Arrhythmias. Contributions include a possible predisposition, surgical (cardiac) damage increasing ectopic activity, salt water aspiration, hypoxia, all the diving stress factors for the sudden death syndrome, as described below.
6. Scuba Divers’ Pulmonary Oedema (SDPE)
7. Marine animal injury. e.g. Irukandji envenomation during initial
dive.
DROWNING
This, or pulmonary oedema of some cause (see previously), is a final event. It was the terminal event, and was verified by the lung findings (lung weights, congestion, alveolar oedema, froth in airways) at autopsy. It is not feasible that drowning could be the sole factor, as something must have led to this. The victim had an adequate air supply and adequately functioning equipment to supply it. Probable contributing factors include:
Panic
Panic contributes to 39% of deaths in our recreational divers (other studies suggest 40-60%). It is a cause of impaired judgment, dangerous decisions, fatigue, over-breathing of the regulator, water aspiration and its serious or fatal consequences.
There is evidence of this lady being excessively anxious, to the extent of panic, both during her diver training – in relatively benign water conditions and at a shallow depth – and following the initial immersion on the day of her death (see Lou Johnston’s testimony).
In this instance there was ample reason for anxiety. Mrs Watson was grossly inexperienced, according to her alleged logged dives, both for depth exposure and existing currents. She was grossly over-weighted, even on the surface, and this will have increased as she descended. She needed the supervision of an experienced and trained instructor.
The decision to return to the descent line, after floating away from it, was not a judicious one, considering her negative buoyancy and the influence of the current. Her failure to make headway, even with her husband's assistance, would have considerably increased her level of anxiety – with good reason.
The statements of Mr Watson regarding the dive, indicating that his wife appeared anxious, supports this scenario.
I understand that she expressed no real inclination for scuba diving and undertook the course to please her husband. If this is true, it is one of the most problematic reasons for such an activity.
Negative Buoyancy
This can be assumed from the excessive weights employed (9 kg, 20 lbs). The correct weights needed to be neutrally buoyant on the surface were under half this, with the equipment she was using. The evidence from both Mr Watson and Mr Singleton indicated that she did not inflate her buoyancy vest. With descent, she would have become even heavier, making both her swimming upwards and against currents more difficult.
A reasonable estimate of her further loss of buoyancy, as she reached the 50 ft and almost 100 ft depth, is 1kg and 2 kg respectively. This is to be added to the already excessive overweighting. Rescue, without addressing the buoyancy issue, would have been very difficult, especially if also swimming against a current.
Negative buoyancy results in; sinking, over-exertion, inefficient swimming, panic, fatigue, over-breathing of the regulator - and thereby increased likelihood of water aspiration around the regulator. It also makes rescue far more difficult – much more difficult than with a neutrally buoyant victim, which is what most trainee rescuers experience in their course.
Salt Water Aspiration
This is frequent in inexperienced divers, especially if they are overexerting or panicking. These situations cause excessive breathing and excessive demands on the regulator. In turn this increases the resistance to breathing and induces the inhalation of more sea water because of impaired functioning of the regulator’s diaphragm and exhaust valves. Also, with the increased negative pressure of respiration, sealing of the mouthpiece becomes more problematic and water seeps into the regulator (which is normally in a dependent position).
The collection of sea water in the regulator may produce a bubbling or “wet” sensation during inspiration, and a nebulised spray is inhaled into the lungs. This has two major consequences. Shunting of the pulmonary circulation across poorly aerated alveoli rapidly results in hypoxia. Any concomitant reduction of carbon dioxide exhalation is overcome by the body increasing the minute ventilation (a natural physiological response to carbon dioxide) – thereby aggravating the over-breathing but not appreciably correcting the hypoxia. The hypoxia induces fatigue, with unconsciousness, then hypoxic spasms and death in severe cases, The cause of death is usually attributed, at autopsy, to drowning. The lungs become fluid filled and heavy with sea water, mucus and oedema.
A somewhat more delayed effect is the accumulation of osmotic fluid in the lungs, from the blood, aggravating the pulmonary oedema, and the other clinical effects, associated with drowning.
Salt water aspiration is a prelude to 37% of the recreational diving fatalities. In some series it is incorporated, incorrectly, as "asphyxia"."
Also, how long it will take for someone to die if air cut off:
"* If by suffocation (obstruction of air supply) >2 minutes (most say 2-3). Death in 3-10 minutes
* This is not applicable to under water (depth, pressure), when it would be longer
* This ignores the ?couple of restricted breaths available after air turned off.
If by replacement of air with an inert gas 1 minute (not relevant here)
Her problem was from aspiration leading to hypoxia, leading to unconsciousness. PROB < 1 minute"
Hope this helps.
