Tom,
I agree with you that there are unfit and unworthy divers out there. As Instructors, we do what we can in the time we have with the students to make sure they ARE worthy to get certified, but we can't always do anything about their diving habits or skill sets after they leave us. I am proactive with the dive shop(s) I teach with to maintain contact with my students after they receive their certs, and I encourage them on the day they get their card to dive with me and practice their skills on the various local dives we do. The shop even allows them to "tag along" with other open water classes, for example. As long as they are diving regularly, they keep their skills up. I realize that this doesn't happen with every student I certify, but the majority of them take me up on my offer and make time to regularly dive. As a result, some of these I have encouraged into doing their AOW or a specialty course, and proceeding further up the training ladder.
All that preface out of the way, I still agree that there are divers who don't keep up their skills, and who maybe dive once or twice a year. As a result, their diving abilities have eroded. I don't agree with your idea of government intervention and having to prove to some government official that you are fit to dive. There are so many ways that that kind of testing can go wrong, and it would be difficult to establish a set of standards that would accommodate every diver's certification level. For example, as an Instructor, you would have these skills already in place, and having to prove it every time you dove somewhere would be tiresome at the least, and annoying at the most.
There have been times when I have been diving in the Caribbean, when everyone in the group had to spend a few minutes at the beginning of the first dive of the trip under the watchful eye of the DM who was evaluating our buoyancy skills. If anyone needed some help at that time, an additional DM was added to the group to work with those individuals to ensure they didn't damage the reef system. This is commonly accepted practice in most resorts around the world as well. I think that is sufficient in most cases.
The exception to the above scenario comes into play in areas like BHB. There is no one to evaluate your skills (hopefully if you are there in a class under the guidance of an instructor, you will be set straight or not allowed to dive!). It's up to the rest of us to try and inform other divers of the potential damage they may be inflicting on the area. Sometimes they are agreeable to this help, other times not so much. I think the good divers outnumber the bad though, and Instructors who have their classes there should be instilling good diving habits into the students' minds.
The SCUBA industry has done a very good job of being self-policing, and the majority of divers who get certified are very good in their skill sets. Having government intervention added to the mix will only befuddle things and create more confusion/paperwork/fees - and don't we have enough of that already in other parts of our lives?
Just my 2 copper-plated zinc discs,