There are several scuba diving training agencies worldwide that certify divers and instructors. The most well-known agencies include:
PADI (Professional Association of Diving Instructors): PADI is one of the largest and most widely recognized diving training agencies in the world, with over 6,500 dive centers and 135,000 instructors globally. They offer a wide range of courses for divers of all levels, from beginner to professional.
SSI (Scuba Schools International): SSI is another large and well-respected diving training agency, with over 2,500 dive centers and 40,000 instructors worldwide. They offer a similar range of courses as PADI and also have a focus on environmental education and conservation.
NAUI (National Association of Underwater Instructors): NAUI is one of the oldest diving training agencies, founded in the USA in 1959. They have a reputation for high standards and rigorous training and offer a wide range of courses for divers of all levels.
CMAS (Confédération Mondiale des Activités Subaquatiques): CMAS is an international organization that certifies divers and instructors, and also promotes underwater activities such as sports and scientific research.
SDI (Scuba Diving International) is a sister agency of SSI, providing similar courses and certification as SSI but with more flexibility and options for the Instructors.
These are some of the most popular diving training agencies, but there are many others as well. Some are more well-known internationally, while others are more regional. It's important to research the different agencies and their courses to find the one that best suits your needs and goals as a diver.