You don't seem to understand exactly what a certification is and what it isn't. In most places, a certification is not legally required to dive. On the other hand, most places that rent or fill tanks as well as most charter boats will require proof of certification before they will fill your tanks, rent their tanks to you or let you dive from their charter boats.
In the early days of SCUBA, people would buy their gear along with a short booklet telling them how to dive. Most would read the booklet and start diving. There were no certifications. Some folks learned on their own more than was included in the booklet and started helping their friends learn to dive. Many, but not all, of these folks learned to dive in the military. Some of the folks issued certification cards they had printed up. Later, they became known as "hot cards." In 1954, LA County developed and started teaching a course in SCUBA diving. This was an excellent course, but reall of no use to those who lived outside southern California. In 1959, the YMCA started certifying divers through it's SCUBA program to become the first nation wide certification program in the United States. NAUI was formed the following year. As time passed, other agencies formed and started certifying divers. Today, there are somewhere in the neighborhood of 30 - 50 agencies that certify divers at one or more levels. SDI is one of the newer agencies. For the most part, all of these agencies recognize the certifications of all the others. For the most part, any place you go to rent a tank, get your tanks filled or go out on a charter boat will require you to have a certification card, but they won't care what agency issued it.
In the early days of SCUBA, people would buy their gear along with a short booklet telling them how to dive. Most would read the booklet and start diving. There were no certifications. Some folks learned on their own more than was included in the booklet and started helping their friends learn to dive. Many, but not all, of these folks learned to dive in the military. Some of the folks issued certification cards they had printed up. Later, they became known as "hot cards." In 1954, LA County developed and started teaching a course in SCUBA diving. This was an excellent course, but reall of no use to those who lived outside southern California. In 1959, the YMCA started certifying divers through it's SCUBA program to become the first nation wide certification program in the United States. NAUI was formed the following year. As time passed, other agencies formed and started certifying divers. Today, there are somewhere in the neighborhood of 30 - 50 agencies that certify divers at one or more levels. SDI is one of the newer agencies. For the most part, all of these agencies recognize the certifications of all the others. For the most part, any place you go to rent a tank, get your tanks filled or go out on a charter boat will require you to have a certification card, but they won't care what agency issued it.
