There are numerous road signs along the way that say "caution" low pay ahead.
When I went "Pro" (spring '01) the course included a discussion about the industry average instructor income being between $21-$24K annually. My first 2 paid days were with a "guided" 6-pack operation out of Key Largo, and the captain and I split over $100 in tips both days.
Having spent the prior year as a Carmel Valley golf resort bartender (~$50K, a third in tips), I knew then I would be able to scratch out a meager living in Hawaii, where all dive boats are "guided" operations. After a few years as an independent/contract/part time instructor I took on the role of "senior instructor" at a whopping $1,900/month salary; exactly as predicted and expected. The small tips almost covered my car insurance (liability only), computers (built my own) and camera equipment (eBay).
When I decided to become an instructor ('97), I was working as a cabana boy, watching the resort instructors pocket $100-$200 cash per day just for the pictures they took of their Intro divers, as well as significant tips. Those kind of resort operations rarely hire green instructors, as few green instructors could handle the job, but as soon as I could I made the resort move.
Between those cabana boy days and my first resort instructor job, PADI took a stronger stand on the Intro photo gig, and the cost of living skyrocketed. Gas prices are 3 times '02 prices, and the room I rent has gone from $700/mo + utilities (~$75/mo) to $1000 + ~$200. How much has dive "Pro" pay gone up in the last ten years? Depending on your perspective dive pay has actually gone down.
Just as in the restaurant industry there are operations where tipping is not part of the business model and there are operations where tipping IS part of the business model. As a consumer, your responsibility is to know what kind of business you are patronizing. A quarry LDS in Indiana paying instructors $24K might not be a tipping business model. Many dive operations in popular dive destinations are some level of tipped business model.
I'm not saying roto-tilling zero to hero's monkey-doing skills on their knees HAVE to be tipped, but I am saying anyone posting absolutes like "instructors are professionals and professionals don't get tipped" or "you made a bad career choice" is very likely an ignorant pot-stirring scrooge!
The only way to keep capable workers is to pay them a "living wage". The dive industry has not been doing that for years and the results are easy to see!