Interesting thread.
I'm a personal injury attorney/trial lawyer in Connecticut.
My answers to some of the issues raised are:
1. If you are a Divemaster and a diver is injured somewhere in your vicinity, there is a greater chance that you will be sued than if you are not a Divemaster. Remember that being sued and being liable are 2 different things.
2. As already stated by Andy, several times, in order for you to be found liable, the person bringing the case must prove that you owed a legal duty of care, you violated the standard of care and that the violation caused the injuries. If someone makes a blanket statement to you that you will automatically be liable for injuries if you are a Divemaster, I say bull. That's simply not true, and it's not a correct statement of general tort law in the U.S. The correct answer is that you may be liable and you may not, depending on the facts applied to the elements stated above and repeatedly stated by Andy.
3. Even if you did nothing wrong and even if you will not be liable, it still stinks to be sued. It costs money, time and stress. So perhaps carrying insurance is a good idea so that you have it to cover: 1) the costs of defending yourself even if you did nothing wrong; and 2) any liability found against you in the event you did screw-up.
4. I understand divers not presenting certain cards either: 1) to fly under the radar in the event of an incident and litigation; and/or 2) to fly under the radar so that on vacation, they may enjoy their dives without being called upon to 'lend a hand' and 'work' during the dives. Just bear in mind that while this has the chance of helping you avoid being 'discovered' as a professional for litigation purposes, it certainly is no guarantee. Competent attorneys will probably find out about your true certification level anyway.
5. Regardless of training, if one of us sees another diver having a problem, we're probably going to do what we feel we're able to do to help. It's simply the right thing to do. In the event the diver-victim or diver-victim's family pursues a case against you as a diving professional, you'll be happy you have the insurance to take care of the lawsuit, pay the costs to defend you and pay any liability if it exists.
6. As I believe a couple people asked, I'm also curious about how many legal claims are brought every year by diver-victims (or their families) and what the outcomes of these cases are. I have not seen anything on this in my readings.
I'm sure I didn't answer every question or address every issue, but I hope this helps a bit.
I don't know for sure, but I bet that these issues are discussed in the SB Instructor Forum which you need to be a professional diver to access. It's just that recreational divers considering becoming professional do not yet have access to all of those discussions in that Forum. Perhaps I'm wrong about that, but if I'm correct, it seems that access to those discussions might prove quite helpful to non-professional divers, especially those who are considering becoming professionals.
Michael