Here's the rule I learned in real estate. Once a professional, always a professional and always disclose the fact to whoever you are talking to. Even if your license to practice is not active.
Granted that the scuba industry is the wild west compared to real estate, but better safe than sorry. For that reason I stopped my scuba development path at rescue diver. An example is a broker lost his RE license for stating the carpet was "new". TREC ruled that the carpet was "used" at the moment the installers walked on it.
I think this probably derives from ethical concerns in a potential transaction. As a realtor, you are in a superior knowledge/experience position relative to others in a transaction, and you have certain ethical duties bound up in your license. If you put it on the table, you are immunizing yourself from claims that you took advantage somehow. I defintely would not follow this practice in diving as I believe it will do nothing but increase your potential for liability. There is nothing analogous to a real estate transaction here -- no potential to take advantage of another person due to your superior knowledge or training -- and so I can see no reason to have to disclose to anyone other than the shop your level of training (and then only the minimum to get on the boat/do the dive).