As it was explained to me, the requirement of Thai employees only applies if you want a non-B visa that allows you to stay in country for 1 year without doing visa runs (this also requires a minimum salary of 50,000 baht/month -- more than most scuba instructors actually make). Maybe what Smartcosse meant by "normal" non-B visa is the non-B visa for which the foreigner must do a visa run to the border every 3 months.
For what it's worth, in Surat Thani (includes Koh Samui and Koh Tao), at least, if not all of Thailand, you can get 2 scuba instructors' work permits per 2 million baht capitalization, not just one WP per 2 million. The diving industry is one a just a few (usually farang-dominated) industries for which the government allows this, as the normal rule is only one WP per 2 million registered capital. My accountant told me that the government knows that a lot of scuba instructors are working illegally, and is trying to make it (a little bit) easier for us to get WPs, hence they will let a company give 2 WPs rather than just one.
All of the above goes with the following caveat: I consulted about 6 different Thai lawyers/accountants about setting up a company and getting a work permit, and I got about 7 different stories as to what the rules actually are. Just one example: about half of the people I consulted said that a company must have 2 million baht IN THE BANK; the other half said "no, no -- the requirement is just for 2 million baht REGISTERED capital. If 61% [not 51%] of the shares are owned by Thais, then you don't need to actually have anything in the bank at all, as long as the company has 2 million registered capital." [I guess "registered capital" means something roughly equivalent to the "book value" of shares in a western corporation, which may have very little to do with the actual market value of those shares]. In any event, I don't know very many scuba instructors who have 2 million baht that they are willing or able to put aside in a Thai bank just so that they can get a work permit ...