"As long as the course meets AAUS specification you'd be on the right path, a recreational vanity program called Certified Virginia Aquarium Diver or something like that solves nothing and creates even more liability, that would be truly foolish."
Many organizations do this with great success. For example, even though I know how to use a fork lift, and have done so with other employers, I do not (cannot) at my present employer because I have not taken and passed their Fork Lift Driver (FLD) course. I even have certificates from other FLD courses; it is not good enough. So, obviously, as a hook to keep volunteer divers, it is something to consider. My Stress & Rescue cert does NOT qualify me to be a professional Rescue Diver. So there are limits to any certification, external or internal.
As for liability, is anyone suggesting that an organization like the Virginia Aquarium does not have liability insurance for it's tank divers, volunteer or paid employee? I think that they, as would any responsible organization with a savvy CFO, have this.