Liquid once bubbled...
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Not True AT ALL!!!!!
In the eyes of the law, if during a dive an accident happen, and beside the DM there's a vocationing instructor, the instructor MIGHT BE HELD RESPONSIBLE IN THE EYES OF THE LAW. I know of several cases. In one the instructor just dived from the same boat with someone that died. It came to court, and his Instuctor licese was taken, on grounds that he was the highest ranker around.
There are a couple of relevant things to say about this:
1) Liability, in most countries involves (a) a duty of care (b) gross negligence in failing to provide the duty of care and (c) damages directly resulting from the negligence.
In the case that an instructor (or anyone) present on a boat witnesses an incident in progress and fails to do the things he is trained to do in order to avoid and accident then he most certainly has a duty of care and has demonstrated negligence and can be sued for it.
His presence on the boat isn't enough, though. Under most legal systems he would have to witness an accident in progress and doing nothing to help.
2) The manner in which gross negligence and damages resulting from negligence are interpreted varies significantly from one country to another.
To illustrate. In America (some states), you can be found negligent for applying a best practice (giving O2 to a DCS victim) simply because the victim didn't give express permission. No actual damage to the victim is required and even if you do it right and the victim has been helped you can still be sued successfully for taking action when you "shouldn't have" (as I understand it)
In the Netherlands permission is implied unless the victim refuses treatment and giving O2 to a DCS victim who doesn't refuse can never be found negligent because you cannot (under Dutch law) prove gross negligence with the application of a best practice (assuming dosages etc are within parameters). Even assuming in some strange case that the applicatoin of O2 somehow made matters worse you'd still have a very difficult time suing for liability because you couldn't establish the negligence. Damages in the absence of negligence is just an accident under Dutch law and accidents can happen without anyone being guilty.
So here you have an example of the same behaviour (rescuer applies O2 without permission) which, under one legal system can be grounds for suing for liability and under another legal system wouldn't even make it to court.
So, getting to the point, It's easy to get all wound up about the "urban legends" and irrational sounding cases but usually these things are taken out of context and sound worse than they are.
R..