This is, as a matter of US tort law, not particularly accurate. As your next sentence gets partly right, the existance of a duty is prerequisite to liability, and in this case one could easily argue that the dead diver in the buddy pair only dove because of his counterpart's training in the buddy system. By holding yourself out as a buddy, you may induce another to dive with you and thereby assume a duty of care towards them that you would not normally have. What level of care is involved there would, in the case of a non-pro diver, be up for debate...but being present when you've both been trained to stay with your buddy would likely be part of it.
OK.....so lets assume that The worlds diving certification agencies all got together and put the thumb rule in writing........You are saying that this would create a LEGAL obligation to follow it? What about the divers that were trained prior to the "rule" becoming accepted and written by the agencies? Would this apply to them as well? NOPE!
You falsely assume that all divers have been trained to dive with a buddy, and to in no circumstances leave the buddy unattended. You also disregard any conversation between buddies, which would have occured during the planning phase that may have negated the requirement to stick together. In the case of a buddy team, where one person is dead.....Who is going to prove that the dead buddy didnt agree verbally to surface earlier because he is an air hog, or that one buddy was going to hang at 20' for half an hour taking pictures instead of surfacing as a team.
All I can say is good luck trying to stick a charge like that against me in court......you wont win, as any person with half a brain can side step "unofficial" liability.....Unless you live in the communist state of California, where a home invader can sue the home owner for a laceration to his arm in the commission of a felony home invasion......
But that is another thread now isnt it?
---------- Post added January 17th, 2014 at 08:21 AM ----------
I can certainly see a legal incident if the individual left alone was a STUDENT!
Agreed, and this also applies to a certified diver that has hired a dive pro as their guide......it creates a duty of care.