If you've found your way to this page, you probably want to know what the difference is between the agencies. The truth is, for an entry level diver, there's some small differences, but PADI and SDI are members of the World Recreational Scuba Training Council (WRSTC). One of the WRSTC’s primary goals is the development of worldwide minimum training standards. As members, both agencies agree to abide by those minimum standards.
There are three primary differences between PADI and SDI.
The first difference is price. PADI is the "big kahuna" of scuba marketing, and as instructors we pay more for almost everything PADI. SDI is less costly for us, and we pass those cost savings on to you.
The second difference is in discretion. PADI uses a prescriptive learning model, which says that certain things must be done during training in a certain order, where SDI allows their instructors more latitude in how training is conducted.
For example, in Open Water training, PADI requires a partial flood of their mask during their first dive. SDI gives instructors the discretion to conduct skills whenever they feel it is appropriate, so an SDI Open Water class might just go swim around, or they might get a bunch of skills done on the first dive.
The final difference is related to the first. PADI prescribes what we will teach in a particular course. Instructors teach only what is prescribed by PADI, nothing else. SDI, in contrast, expects their instructors to add to every course they teach. SDI supplies the minimum standards, but instructors are encouraged to make courses more challenging and more interesting. Our SDI courses cover everything in the PADI curriculum, but we teach other skills on top of those.