But again, who’s fault is it with the current situation?
What is it about teaching that has them lined up willing to work for next to nothing?
The artist industry frequently runs into a similar problem. People love creating art so much, that many new artists are tricked/manipulated/convinced into working for next to nothing.
There's also somewhat common (IMO bad) advice to "do what you have to, to get your foot in the door." Meanwhile, there are many businessmen types who know this all too well, and will approach one-artist-after-another until one of them agrees to work for free. Typically they'll say something like "think of all the publicity you'll get", leaving out the part where that publicity is approximately worth the $0 they're paying you.
How do you break this cycle? Generally, this is where I've had the most success:
1. If you ever work for free, it must be pure charity or probono. You're better off building your own brand and client-base, perhaps offering a free finning lesson or equivalent, than getting almost nothing from a shop, whose charging customers hundreds of $.
2. Be a "mercenary." Employers aren't your friends, even the nice ones. Work for whoever makes you the best offer, in terms of what they pay and what is expected in return. Seek and take better offers as they become available. The only person who genuinely looks out for your best interest is you, even if your boss does a damn good job pretending to be your best friend (even if they genuinely mean it).
3. Consider freelance work and cutting out the middle-man. There are some obvious complexities here, you have to do your own marketing, book-keeping, etc. It's worth doing some reading on running a freelance operation. I don't know about any scuba-specific books, but even the ones I've read designed for artists generally have a number of useful thing to learn. For example, if running your business, your cost must also compensate for time and resources spent outside the class itself.
4. References are money. References are how you move from crappy-job, to less-crappy job. References are how you get clients that recommend you to other clients. References are the way to avoid spending exorbitant amounts of money on advertising. This is true of freelance or even normal jobs. 3b: Make trusted friends in the industry. When you're over-booked or get requests for a class you don't teach, send potential customers to people you trust. Ideally, that person also sends customers your way, when they're overbooked or get requests outside their specialty.