Scuba diving is one training-intensive hobby. Having a scuba certification card from a reputable scuba association assures the people manning different dive shops that you have been trained to dive safely and responsibly. Dive shops will require you to present your scuba certification card before arranging a chartered diving expedition. It is not just because they are concerned for your safety, but because inexperienced novices could ruin the diving experience for everyone.
Certification is also required when renting any equipment from dive shops, as they can be damaged if not used or handled properly. Proof of your training includes returning them in good condition.
Scuba enthusiasts always say that getting certified is harder than getting a library card, but easier than getting a driver’s license. It is not easy, and most certainly not free. You are required to prove that you have the know-how about using the equipment, handling yourself underwater, and knowing how to react in various underwater situations.
There are many scuba certification programs today, the most popular of which are PADI and NAUI. The former is considered the “lighter” program, making it ideal for tourists who only want to take a few supervised dives; while the latter is more involved and ideal for a beginning wanting to take up scuba as a recreational sport. After completing the beginner level, both programs offer advanced-level training.
Source: The Scuba Guide