I got certified on a whim while on a tack-on vacation after an exhausting three-week public speaking tour. I went to a beach resort just to lie around and relax, and within 24 hours I was bored--with four more days of free time ahead of me. I don't know if it was the relative quiet after being surrounded by huge audiences every day or if it was the entirely different environment of the underwater world that made me curious to experience more of it, but learning to dive was like an epiphany, and it took my life in a new direction. I've always been an educator, so teaching scuba is more of an extension of my vocation than a complete reversal, but this gives me a chance to share something I really love. These days I have (mostly) disengaged from my previous career to focus on scuba.
I set up my school at a busy tourist destination, and I learned that many of my colleagues get burned out pretty quickly with the long chain of four-day intensive classes with four to eight students in them, so I decided to teach only privately, by prior arrangement. That way I stay excited about it and don't wake up and wish I didn't have to get in the water. Just teaching, though, isn't very lucrative, so people who do stick with it need some other source of income--a "regular" job, a dive shop that sells gear, or a dive travel service (and I suppose sometimes all three). I teach some and I help people plan their dive vacations in the region (Thailand/Malaysia/Indonesia), for which I receive commissions.
I plan to continue teaching. I get a reasonable number of repeat students considering that they have to travel quite a distance to get here, so that makes me feel that I've been successful in passing on my enthusiasm for scuba and instilling good skills and habits in my students.