The main problem, as I have seen it in business here before, is the Thai requirement that there must be 6 Thai workers (yes, I recall the ratio as 6 to 1) for every one foreign work permit. The exception is when you are the sole managing director of your own company. Even, then there would need to be 6 Thai employees (full time) to get a work permit issued for the second foreign instructor. If there were 2 additional foreign instructors plus the foreign managing director, then I am pretty sure that their must be 12 Thai workers on the payroll.
Also, I don't think the laws vary from province to province in Thailand; what varies, from what I have seen, is the "under the table" influence by Thai agents who have "friends" who work for the government, and as we all know, the game changes when you have an "agent" as a go between who has a connection, friend, family member, etc.
I have seen work permit applications from three companies and all were the same, basically, and all had to show the business (photos) and provide details of the company finances, number of Thai employees (on the social security roster), etc. The (correct) paperwork is very extensive.
I have had three different Thai work permits (before I stopped working completely before doing my "retirement" visas) and the process was just about the same each time.
On the other hand, I don't want to get up on a soap box and judge others and their integrity based on how they operate in Thailand with regards to immigration and work permits. Let's just be clear that most dive centers have people "studying" on ED (and sometimes tourist) visas and, from a Thai legal perspective, they should not be working, but in practice, I don't see that it stops many people, as RB said above.
If all the foreign scuba instructors in Thailand stopped teaching that did not have a proper work permit, I would guess the number of instructors would be dramatically smaller, with the exception of the 'student-instructors' who are "assisting" as a part of their overall "education".