Just to add a few comments....
Phuket is strict, whilst other areas I agree are less strict.
I arrange work permits for people in Phuket that ask me for them, however, this year, the Phuket labour office has been clamping down, and in addition, TAT (tourism authority of thailand), have been enforcing that they are including in the money collections too. TAT say that if you're dealing with tourists, you must be tat-registered. This applies if the freelancer is running through a company setup for freelancing. Like mentioned in posts before, there are not many dive centres that will issue work permits through there own company to employee a temporary diver.
The amount of paperwork involved in preparing a company formation is very low. The amount of paperwork involved in preparing for labour office and the tat office are very high.
Last year, getting work permits tool around 6 weeks, this year they're taking much longer and the offices involved are being difficult to deal with. It's becoming more and more clear that whilst incoming divers want to work legally, there is no clear legal way for divers to work legally in the current legal system. A new legal working route really does need to be made for divers and the way that they work for other dive centres as required (freelancing).
Work permits state the allowed location(s) of your work to take place. If you're found working in places other than documented, and although you have an actual work permit, it's still a technicality that can be used against you.
It's fast becoming extremely risky to put up so much cash, to earn so little in return compared to the number of hours a diver generally works.
If anyone is looking for permits or accountancy services in Phuket, I will share my contacts and indeed help you myself, if I can.