I admit it. The articles I write for The Diving Blog tend to be PADI-centric. Do I think PADI is better than all the others? Not really. I did my Open Water certification with PADI and just happened to move through their system. I'm working on a PADI divemaster certification (and soon, Open Water Instructor).
To introduce a little fairness, I decided to investigate several agencies and report on the differences. The result may not be a big surprise. The differences between agencies are small. When you dig deeper you uncover the reason for this. There are international (and national) standards for certification requirements. That is, representatives from different agencies agreed upon the minimum knowledge and skills required for each certification level.
This has two implications. First, it means scuba training across the board is going to be very similar. Fluctuations in instructors are more likely to determine your individual experience. Second, it means most agencies follow the same certification hierarchy: open water diver, advanced open water diver, rescue diver, divemaster, assistant instructor / instructor. Names may very slightly, but the overall structure is the same, with each of these courses following the standards set forth by ISO 24801-2 (open water), 24801-3 (divemaster), 24802-1 (assistant instructor), and 24802-2 (instructor). The important thing here is that most standard-respecting agencies will respect comparable non-professional certifications from other standard-respecting agencies. Sometimes a peek at your log book may be required, or worst case, an accompanied dive to demonstrate your skills. Call your destination's dive shop ahead of time if unsure.
There are a lot of certifying agencies. For my research, I looked into those that are a) international, and b) the largest. Here some information about each of those:
PADI is by far, the largest certifying agency in the world, with over 5700 dive shops and resorts in over countries. It was founded in 1966, by effectively splitting off from NAUI out of frustration.
PADI revolutionized scuba instructor by turning it into an hobby for everyone, rather than elite swimmers and athletes. The learning material reflects this, by breaking down every process into distinct pieces for easy consumption. This approach to learning extends all the way up the ladder to divemaster and instructor levels. Instructors are expected to adhere to every aspect of the PADI system, not only what is taught, but how it is taught. This makes the PADI system incredibly consistent across the globe.
Being the largest certifying agency (issuing two-thirds of all certification cards in the US each year) comes with its criticisms. First, is how PADI constantly attempts to upsell its customers. The entire last chapter of the Open Water manual is an advertisement for Advanced Open Water. Almost the entire divemaster manual says why you should immediately go on to the PADI instructor-level. PADI is a for-profit corporation, and this sometimes reflects in their material, with many jokingly referring to PADI as, Put Another Dollar In.
The second criticism, however, has less of a basis. Many claim that PADI "dumbs down" scuba diving for mass consumption. It is true that they break the learning process down into its simplest components, but this can hardly be considered a bad thing. This elitist attitude neglects the advantages that have come from the boom in scuba diving, namely easy access to dive sites and equipment practically anywhere in the world.
NAUI is the oldest certifying agency in the world, established in 1959. NAUI even boasted Jacques Cousteau as a member of its advisory board at one point. Another distinction belonging to NAUI is that it is also one of the few non-profit agencies (the only big one, as far as I can tell). NAUI's reputation has earned it a spot as the certifying agency for many colleges and universities, the US Navy, and even NASA.
While not a member of Recreational Scuba Training Council (RSTC), NAUI is ISO certified.
SSI was founded in 1970, and has over 2400 offices in 110 countries. SSI takes a unique approach, in that training is only provided through one of their worldwide centers. Despite this centralized approach, SSI gives its instructors more freedom in how material is presented, giving instructors the opportunity to adjust style based on student participation. I suspect their centrist policy allows this freedom, since instructors are teaching within a dive center where styles and student reactions are easier to monitor.
SSI emphasizes its teaching style of "comfort through repetition." I don't see how this is much different than most agencies, but the most noticeable result is that SSI requires five open water dives for basis certification, rather than the four required by the standard (and followed by most agencies).
SDI is the recreational arm of Technical Diving International, the largest tech diving training organization. It is also the baby of the group, recently founded in 1999. Despite getting a late start, SDI now has over 2200 authorized dealers worldwide, putting it just a hair behind SSI in size (although I don't know the geographic distribution of either).
While a standards-recognizing agency, SDI is unique in two ways. The first is immediately noticeable in their Open Water course: all students are required to have modern dive computers. SDI gets students used to diving with computers from the start. This could be seen as an advantage, since it eliminates the need to teach dive tables and introduce further confusion. I have also heard this as a criticism, since should a computer fail, the user is left with no alternative means of dive planning.
Secondly, SDI is the only agency that offers a certification for diving alone, Solo Diver. Every other agency mandates diving with a buddy at all times. I imagine this aspect of SDI was influenced by its origins as an offshoot of TDI, where diving alone is normal.
If one of these stands out to you, or you find their teaching philosophy appealing, then go with it. Otherwise, don't fret about any particular agency. Instead, spend your time finding high-quality instructors, either through recommendations or online reviews.
Is there any other reason you would suggest one agency over another? What is your experience diving abroad with the smaller organizations?