What is the difference between PADI and SSI?

PADI means Professional association of diving instructors and it's especially renowned worldwide. Many divers even say “I have my PADI” to state that they are certified. SSI stands for Scuba Schools International. PADI and SSI follow the diving standards set by the Governing body – WRSTC (World Recreational Scuba Training Council) meaning that knowledge and skill requirements are almost the same with only small differences. The certifications are fully interchangeable too.  At the end of the day, everything depends on personal preferences, and for example, on the experience and willingness to learn from your instructor. Moreover, the professionalism of the dive centre also plays an important role.