This page is a list of know certification bodies or associations.
Certification is very important for safety, insurability, and revenue use.
For Certification your sub will have to get inspected and be build to a set of standards, this enhances safety. If certified by a standards organization you can get insured since the insuring agencies know your sub passed inspection and comply with a know set of standards. For revenue use by contracting out to a business, private parties, or a government agency you will need insurance which means certification. Of course if your sub is strickly for private use you can dispence with certification. However this can be risky and you should at least use their standards.