What are the names of some of the certifying agencies and why are they different?
The agencies formed independently, each developing its own curriculum and, while there are many similarities, they are not all identical. Some of the most popular agencies are known by their initials: IDEA, MDEA, NASDS, NAUI, PADI, SSI and YMCA. Of this group, YMCA is the oldest and PADI is the largest.