I don’t have a problem with any level of education from one of the agencies. I take them all with a lot of salt, if I don’t know you and what you can do, I don’t care what your card says or how many of them you have. It is the quality of your instructor and your experiance with the use that instruction that means the most.
As for the term Master Diver – well:
In PADI it is a congratulation, you have passed 5 courses, all of your checks have cleared, and we got you to pay an extra $50 for this nifty 6th card.
Master Scuba Diver Rating Courses - Advanced Open Water Diving - PADI Scuba Dive Training Organization
In NAUI it is a certification and you may, or may not get additional education. In other words, it’s not just sending in a 6th check and I hope that your Instructor does give you additional education to earn it.
NAUI Worldwide Master Scuba Diver
Overview
The course is an continuing education certification course for divers who wish to increase their understanding and enjoyment of diving. Emphasis is on student participation and practical application of knowledge in open water after a classroom discussion of subjects. This course is an excellent progression toward NAUI Leadership roles.
Some subject areas are a review and expansion of material from previous courses. Each subject area is a progression in study not a definitive study in the particular diving activity. However, instructors will specify performance objectives for related course diving activities, for example, during a navigation dive the student will swim a reciprocal course to within 10 feet (3 m) of its origin.
The course may be divided and taught in sections with the student’s Diving/Training Log being signed off for each activity until all requirements are met.
Upon successful completion of this course, graduates are considered competent to engage in open water diving activities without supervision, provided the diving activities and the areas dived approximate those of training.
The NAVY
Never talk about being a Master Diver around US Navy divers. A Navy Master Diver is a very experienced diver and has worked very hard for 10-15 years to get the Rating. They will also be a Senior or Master Chief Petty Officer by the time they rate and earn a Master Diver qualification.
Navy Diver : Special Operations : Careers & Jobs : Navy.com
So to summarize, a Master Diver is:
PADI – A Congratulation
NAUI – A Certification
NAVY – A Qualification and Rating