Dive instructors are typically the poorest, happiest, people you'd meet. If you need money, then it isn't a wise career choice.... and we all need money sometimes...
Most scuba instructors have a short career-span in diving - a couple of years, often coinciding with an academic gap year, or career break. That is followed by 'part-timers', those who work mon-fri in a 'real' job and teach diving as a hobby. The next smaller demographic are mostly retirees, doing it as a passion. The tiny percentage of instructors who retain a long-standing career in diving are normally the owners of dive-related businesses (i.e. dive shops) or have some other form of secondary income to sustain them over a long term. Last, but not least, are the tiny percentage who have acquired sufficient experience and qualifications to make a living as full-time instructors, normally in specialist (typically technical related) diving pursuits.
Military, police, commercial and scientific diving is another matter.
For public services, including military, you're paid for your primary function (i.e. a policeman), with possibly a small bonus to reflect your specialism.
Commercial divers can earn big bucks, but the cost of training is very expensive. It's also rarely described as "fun" or as having much appeal beyond the financial incentives... it certainly doesn't equate to any experience you might have on holiday...
Scientific divers, who work in a field necessitating underwater research (marine biology, underwater archaeology etc) are more likely to receive a more attractive long-term income... based on their academic/scientific merits. The diving is a means to an end - but that doesn't preclude it being enjoyable.