I would agree that it's a good idea for more people to take a CPR class. CPR is one of the most basic first-aid responses that can have a significant outcome on someone's life, and the course only takes about 8 hours or so. Since my career lies in emergency medicine as an EMT, and soon to be a paramedic in September, I believe that CPR should be taught to every child at some point during elmentary school, and then refreshed again in high school. That way more people will accept is as common knowledge.
Now, with that being said...CPR is huge in the "chain of survival" which is taught during American Heart Association (AHA) courses. If you know CPR and do end up having to use it, don't be surprised if the EMS crew has you continue to help even after they get there. There is a lot to be done by an EMS crew during a cardiac arrest. You just could be part of saving someone's life. You might not save everyone, but at least you can say that you tried your very best.
As far as the legality of everything goes, it depends on the state that you live in. If I'm in my personal vehicle and see an accident, there is nothing that says that I have to stop in Connecticut. I can just keep on going. In order for me to get in trouble, there has to be a "duty to act." Here in Connecticut, that consists of someone activating EMS through 911 and me being dispatched to the call. I have a moral obligation to act in the best interests of those involved, but that does not translate into a legal obligation.
Don't be afraid to learn CPR. I saw a poster just tonight at work at one of the hospitals that said it well. "Hero's are not born, they are trained. LEARN CPR." I thought that was a very well put statement. If you learn it and the chance comes up to use it, you're education will just kick in and you'll start to do things without thinking about it. You may have to think a little more about what you're doing then someone who does it on a regular basis (such as a police officer, EMS provider or firefighter)...but something is better then nothing and gets the process going.
I have not taken the rescue diver course yet, but I do plan to over the summer. I just recently obtained my open water cert, and the rescue diver and advanced open water are in order before moving on to my public safety diver. Paramedics even cover some diving emergencies in the "environmental" portion of their education, but more education and skills certainly can't hurt. Maybe when I go through the class, I'll be able to help people by giving some insight from the side of being an EMS provider? There are other conditions that can closely mimic DCS. As an EMS provider, I can't assume that because someone has been diving and has signs and symptoms of DCS, that's what it is. I can strongly suspect, but there are things that we have to rule out during our assessment. If you have the training, give oxygen. Oxygen won't hurt someone, especially someone medically cleared to scuba dive.
If anyone has any questions with regards to CPR or EMS, feel free to leave me a message.
Shane