I think with the greatest respect to the other posters you have to look at it from the perspective of being in Thailand.
You have also got to crunch the numbers and see if what you can earn will be enough to live on.
There certainly seem to be a number of independent instructors who make a decent living out here.
To look at some numbers...
What does an Open Water student cost an instructor. (all US $)
Rent of a pool and gear for 1 day $5
Class room free (any quiet place will do)
Course materials almost free (they all seem to be borrowed
or copied)
Cost of 2 days for the student on a boat (inc gear) $50 (instructor normally goes for free if he brings a few people)
Pic $15
Other costs travel , padi dues etc $10
So total costs 5+50+15+10=$80
I think if you look around the web the min. price for OW is about $300 so give or take a little the instructor is earning about $220 a student.
So if you get 4 students / week we can call that 16 / month thats over $3,500 / month. Thats a heck of a lot more than you get flipping burgers in Thailand.
If 16 students / month seems a lot you should also consider that if you can just talk somebody into going for a days diving them your cost is $25 and they will pay you $75 (check prices around the web) a handy $50 / person / day without breaking a sweat.
All these facts and figures mean little if you are unable to network and go out and find customers.
Working for one of the larger 5* centre's will certainly not make you a million bucks. They have overheads such as building rent , capital outlay on Boat + Gear that needs to be taken out of the income before you start thinking about the centre's pound of flesh. You would probably beat a burger flipper , but not by much.
Minimun wage here does not beat $5 / day. But I think a college degree is now min. requirement for flipping burgers.
You certainly can make it work and certainly live very comfortably if you are prepared to put in the time and effort and you need to have the right kind of personality for it.
One think that is worth mentioning is that you would be earning all this tax free
but that obviously means you are working illegaly:mean: