I realize this is somewhat off the subject by the OP, but frankly it is a more interesting and germane subject for anyone considering dive leadership certification.
In US law, duty to act is an evolving subject. You are correct that absent a legal duty to act, no one has a legal obligation to respond. That may shield you from criminal liability. But it does not necessarily protect you from civil suit. As a trained EMT, you are off duty, having dinner with a friend who begins choking. You refrain from acting and they end up gravely injured. You get sued and stand on your absence of legal duty to act. Now consider the deliberations in the jury room. I wouldn't want to be your lawyer, even in a 'no duty to act' jurisdiction.
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see firefighter.com for more on this isssue
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Excerpts from 'NAUI Leadership and Instruction' (pg. 169) on STANDARD OF CARE AND DUTY: "With the privileges of leadership status comes the burden of responsibility for the safety for those divers with whom you interact. You have a duty to act in their best interests according to the recreational diving community's standards. Such criteria are not only formally established by NAUI, but also by the generally approved standards, procedures, and customary expectations of recreational diving leaders and other diving organization around the world.
Though diving leadership practices will obviously vary from place to place and evolve over time, every NAUI member assumes a duty to act with care, reasonable judgement, and diligence when dealing with students and divers.... This duty may well arise even though there is no written contract between you and the other divers with whom you associate, and even if you are not being compensated for your efforts.... If an accident occurs, questions of blame, as well as discussions about what was prudent and reasonable for that diving event or training situation, will inevitably arise."
I'm not a lawyer so please don't construe any of this as expert advice. I'd be interested to hear from anyone with actual experience in 'duty to care' issues.
Stay wet... Avra