To lend my own thoughts and experiences to this analysis; I have been part of two near-misses which are strikingly similar to this scenario and have some experience with Add Helium and the diving practices Rob would LIKELY have been employing.
Near Misses:
1) While conducting a Navy CCR dive the diver failed to monitor his PPO2 and due malfunctioning solenoid that failed in the closed position the diver went unconscious upon ascent. The diver was on a tending line and signaled that he was ascending but then failed to respond to subsequent signals. Within seconds of sending the last line signal we saw him floating unconscious within 10 feet of the surface. We recovered him into the boat and began resuscitation. He was not breathing, his skin was completely blue, and we could not discern a pulse (it was a difficult sea state). Within minutes of beginning rescue breaths he began agonal breathing and by the time we were back at the dock (15-20 mins) he recovered and was alert. He was diving a MK16 with a full face mask, without the full face mask I’m sure he would've drowned.
2) Conducting a civilian CCR dive on a chartered boat a CCR diver had issues with his rig just prior to entering the water. The diver removed his rig, removed the scrubber, and checked the cable connections. The diver put the rig back together, put it back on, and entered the water - never having done the build or prejump checklist. He never turned on his 02 and went unconscious on the surface due to hypoxia, the loop fell out of his mouth, his rig flooded but maintained buoyancy due to an inflated wing. Another dive boat actually saw him unconscious in the water as he was under the gunnel and not visible to the crew. The crew recovered him and were able to resuscitate him. He was not diving a gag strap.
Diving practices likely employed by Rob:
1) Add Helium advocates the removal of the gag strap to facilitate a bailout at depth. Of all the diving maladies associated with CCR diving specifically with managing a breathable gas supply (hypoxia, hyperoxia, and hypercapnia), hypercapnia is only one that is not measurable and has no effective instrument to provide warning. As such many CCR divers choose to remove their gag strap to facilitate a bailout in the event of hypercapnia. Rob was likely not using a gag strap, as this is the procedure advocated by Add Helium.
2) Rob likely would of kept the loop in his mouth upon surfacing and for several more minutes even once aboard the boat. Continuing to breath a high PPO2 upon surfacing is optimal for decompression and is something taught by Add Helium.
3) Rob was likely diving a 5mil semi-dry and not a full dry suit which would of provided additional buoyancy. Add helium promotes and sells the Semi-drys and Peter Sotis dives them. There is a picture of Rob in one days before the dive.
My hypothesis is that Rob went unconscious on the surface due to hypoxia, subsequently the loop fell out of his mouth resulting in a loss of buoyancy and he ultimately drowned. Barring a physiological issue (heart attack), going unconscious from hypoxia and the loop falling out of his mouth (open) would explain why an experienced, in shape, diver would suddenly disappear from the surface.
