Qualifications
Before you can teach anything that remotely resembles SCUBA diving, you'll need to get the appropriate certifications. Two major organizations certify most of the world's instructors, and we'll briefly summarize each company's general requirements.
PADI, or the Professional Association of Diving Instructors, maintains affiliations with over 5,000 instructional facilities around the world, including dive shops and resorts. PADI operates on the principle that, as you acquire more skills and certifications (through PADI, of course), you receive approval to teach higher-level classes. Sounds like a nice revenue stream for PADI, right?
You should also know that all PADI facilities are not created equal (source). For example, you can complete the Divemaster course at several different types of PADI facilities. However, you must take your Open Water SCUBA Instructor course at a PADI Five Star Instructor Development Center. It's way more detailed than we can discuss here. The PADI website has a nice, clear table that shows the courses you can teach with different-level certifications.
Next, let's talk about NAUI, or the National Association of Underwater Instructors. NAUI has its own structure for certifying instructors for NAUI's dive courses. The two-part Instructor Training Course includes an instructional segment and an evaluation segment. If you're a PADI instructor and want to teach NAUI courses, you've got to complete a specific course that teaches you the NAUI standards. Yes, this is a nice revenue stream for NAUI, too.