If you're REALLY luckily teaching scuba you may make enough money to break even.
And things like working in a dive centre and tech diving with a bad back to me sound like a very bad idea.
Sorry, but it's gonna be a lot of words......
The first statement is not true, I've worked full time as a SCUBA instructor for 12 years now, and done all right. Not rich mind you, but paid all my bills and had some spending money. Ive been able to afford a vechile (when I chose to own one) and to go on vacation once or twice a year. It does depend a whole lot on where you work. I'd say you could expect to make between $25,000 and $35,000 USD/year (that includes tips and bonus), possibly including accommodations.
You will probably get health insurance, but have to pay your own liability insurance and instructor fees. Youll need your own gear, but youre going to get special prices on gear so thats ok.
If you want to work at the prime dive locations Id suggest getting a PADI instructor course over NAUI. Ive never seen a NAUI shop in the Caribbean and dont know of any here in Hawaii. Ive worked with NAUI instructors, but only 2 or 3 (in 12 years) and most of them had a PADI crossover. Im not saying either agency is better or worse, Im just saying that 99.9% of the jobs Ive had/seen advertised required a PADI instructor.
Now to be fair, I'm moving out of the SCUBA field to (hopefully) make more $, but it worked just fine for me, if I had to do it all over again I wouldnt change much. I've lived in fantastic places (twice directly on the beach), dove in the best locations, seen the best stuff. So even though the pay is low compared to say an engineer, you be surprised (or perhaps not) how many people in the SCUBA industry used to have white-collar jobs before moving into the field.
The second part: You can't have a bad back. You maybe able to take the instructor course, but as far as working goes, it won't go well. You have to realize that working as an instructor is not all stingrays and turtles, it'sgenerally a very physically active, lifting oriented job. For example to prepare for a boat trip (of which you'll generally do 2-3 per day, 5 or 6 days a week) in a tropical dive location (why would you want to work as an instructor anywhere other than a primo dive location?):
1. Load the boat with tanks (anywhere from 10-40 tanks per trip, depending on the boat).
2. Load the gear for the divers (again anywhere from 5-20 setups per trip)
3. Setup the gear.
4. Setup and distribute weights (like a 10-20 lb belt per person)
5. Tie up the boat/anchor
6. Possibly wander around the (rocking) boat in full gear fixing minor problems.
7. Go for a dive, generally carrying 2-10 lbs worth of extra weights.
8. Exit the water quickly, carrying your own gear (sorry but there's no way the capt/deck hand have time to deal with your stuff).
9. Pull anchor/untie boat (I've never worked on a boat that had an electric winch, anchors will weigh from 20-60 lbs + chain and line). Despite the environmental issues, Ive worked on many boats where the anchor was used over a mooring.
10. Change all the gear in 10-20 minutes (while the boat is moving).
11. Repeat 5-9 for dive 2
12. Return to dock, tie up boat, and unload all tanks.
13. Fill tanks (which involves a lot of tank moving).
14. Do it all again, probably without any sort of break.
That's all assuming everything is ok. You may need to tow/rescue someone who is up to twice you size. Working at a beach diving op is no better. You remove the boat stuff and replace it with sand, which you have to carry gear/tanks across.
Now I have worked with people who have had back problems, but it never lasted more than a month or two. One issue that you'd need to overcome is the balance of work between you and the rest of the crew. Everyone who I've worked with that had back problems ended up getting all of the junk jobs (clean the head, etc) because they couldn't help with the tanks (a major part of the work day). They got passed over for the special trips because those trips usually had a smaller crew and those folks needed to do everything.
So Im not saying that it cant be done, but I am saying that it would be hard. If you do go for it, be choosy about the jobs you take. Ask lots of questions and find out where youre going to be doing the least lifting. Some shops (Ive worked at 2 now that did this) had their compressor setup in a location that it could fill the tanks while they were on the boat (saving everyone a LOT of lifting). Other shops may have tank monkeys (weve got a guy now whose sole job is to deal with tanks
sweet).
But no matter what, you have to be able to rescue big people. If you cannot do a full rescue from boat or shore on someone who is literally twice youre size, you simply shouldnt work in SCUBA. While Ive never had to pull someone from the water (thankfully), I have had to tow 300 lb panicking divers 300 or 400 yards back to the boat, followed by the resumption of the schedule. Its part of the job.
I hope all the above helps you in youre decision.