I recently signed onto this site and I really like it. I have seen a few discussions on the topic of liability in event of a rescue. 'Duty to Act' only applies if you are recieving compensation (e.g. paycheck or charter fees as a rescue diver/specialist. Even then, the rescuer is not required to put their life in unreasonable danger. That would only serve to make 2 rescuees when rescuers with more training and equipment arrive, thereby forcing the resources to be divided and less effective.
When giving aid, give the best care you can within your scope of training. DO NOT ATTEMPT anything you haven't been trained on. The Good Samaritan Law varies from state to state, but effectively eliminates any threat of a successful lawsuit SO LONG AS YOU ARE NOT RECIEVING COMPENSATION! Check for your local Good Sam Law to be sure, but in CA it states that the rescuer cannot be held liable for any act or omission in the performance of emergency aid given in good faith and not for compensation.
Hope this helps. Incidentally, any rescue course you take will be one of the most demanding courses of your diving carreer, but also one of the most satisfying.