dharmon
Registered
I think if you compare the requirements for a C-card you will find that there are differences. What one agency says is enough may not get you qualified by another.
Most definitely, I'm just saying there are "core" skills that have been agreed upon for different certification levels. For instance, you'll learn how to clear a mask in a course from any agency.
For PADI, go to this page, scroll down to see where they tout being standards-compliant.
For NAUI, look on their Wikipedia page.
I don't have the info for any other agencies readily available, but these are two big ones.