I have been quite on this thread, hoping to get more information from both DAN and the coast guard. Unfortunately, I am still waiting. I knew Don, he was a friend and my animals veterinarian. I am a USN/NOAA/UHMS diving medical officer that teaches dive medicine to other physicians. The possibilities of the initial cause for an accident of this type range from Cardiac ischemia (heart attack), IPE, inert gas narcosis, oxygen toxicity, hypoxic mix, CO poisoning, hydrocarbon/oil poisoning and/or hypothermia. Most of these have been ruled out. Without gas analysis of his tank it is impossible to 100% rule out CO, Hydrocarbon/oil, hypoxic mix, Oxygen toxicity. I would lean towards IPE or myocardial ischemia as the most probable cause. I did not see how much rubber Don was wearing or if he had gained weight since purchasing his wet suit. Cold and tight wetsuits can bring on IPE. The "AGE" could have occurred during the ascent, especially if he was not breathing from 20 feet to the surface. The"AGE" also could have been off gassing of nitrogen from the dive after he expired and not clinically relevant.
Hypoxia, either from low cardiac output or pulmonary edema could broduce the blank stares, shaking and eventual seizures that were described.