Let's be careful about this. I am a regular reader of the DAN fatality studies, all the way through to the individual case descriptions. Each of the above is true to a certain although differing degree. First recognize that the primary factor in scuba fatalities is related to cardiac health, so we are talking about the much smaller percentage of fatalities related to errors in diver procedures
Panic is definitely a major issue--probably the most important. The joint PADI/DAN study on fatalities a few years ago identified embolisms following panicked ascents following OOA incidents as the primary preventable cause of scuba fatalities. I also attended a scuba insurance workshop that essentially said that going OOA rarely kills anyone--it is the failure to follow training after going OOA (taking that panicked flight to the surface) that really causes the problem.
Overweighting is sometimes a factor, but it is a much smaller factor in comparison to panic. In nearly all such cases, the problem was again related to being OOA. The divers made it to the surface successfully but were unable to remain there. The divers neither orally inflated their BCDs nor dropped their weights. It is for that reason that PADI put extra emphasis on both oral inflation and dropping weights in an emergency in their new standards.
But what does that have to do with being overweighted? It is a logical inference from such a situation. In a standard weight check, the diver is supposed to be able to float at eye level at the surface with an empty BCD and without kicking. A diver who has reached the surface following an OOA situation should have a fair amount of air in the BCD already, so a properly weighted diver with an empty tank should not only be able to stay at the surface without much effort, that diver should not be able to sink. A diver who cannot stay on the surface with a partially filled BCD and an empty tank must be grossly overweighted. In a horrible case well detailed in a ScubaBoard thread a few years ago, a man wearing a new BCD was hunting for lobster in Florida when he ran out of air. He got to the surface but could not remain there because he was grossly overweighted (apparently intentionally to assist him in his lobstering) and could not figure out how to ditch the weights in his new BCD. A woman using rental gear tried to help, but she was unable to help him stay at the surface. Because her rental gear did not include an alternate air regulator, they began to buddy breathe from her one working regulator, and they both drowned.
In summary, panic is indeed a major factor in preventable scuba fatalities, and being grossly overweighted (not 2-6 pounds) is one factor in a cascade of failures in a much smaller number of cases. In every case I can remember in which being overweighted was a contributing factor, the diver was OOA on the surface before death. Since many and perhaps most recreational divers are overweighted to some degree, many and perhaps most fatalities feature overweighted divers, but it is hard to identify being overweighted as a contributing factor in the fatality.
