There are dive professionals making far less than minimum wage right now. I frequently made far less than minimum wage often, and that was when minimum wage was about $7.00/hour. I would bet that the pay scale in that shop has barely moved since the legal minimum wage doubled. They can do that because the instructors know that if they complain, they will be out the door and replaced by somone else dreaming of the good life as a scuba instructor.
Another reason they can get away with it is their own ignorance of the law and their vague means of paying people. For example, I was paid by number of divers and the dives they did, with no reference to time. They would say, well the dives should take this long, and you have this many students, so you made minimum wage. You would then point out that you had to arrive at the shop, fill the van with equipment, drive 45 minnutes to the dive site, unload the equipment, brief the dive, gear up, do the first dive, get out of the water, do the surface interval, clean up, put the gear in the can, drive back to the shop, unload the van, put the gear away, and do the paperwork. They would say that none of that other stuff counts--only the time actually diving counts for time being paid.
That lack of reference to time is magnified on long trips. For example, if you are in the Denver, CO area and conduct OW dives in Utah or New Mexico, you have 8 and 7 hour drives to and from the dive sites plus overnight motel stays, and none of that time counts.
In one conversation about this with the Director of Instruction, I computed my hourly rate for the last assignment at just over $2 per hour. He said that was perfectly acceptable under the law because tipped employees can get paid that amount with the hope of making up the difference in tips (which are rare in scuba instruction). When I told him that according to the law, if the employee's tips do not get the pay to minimum wage, then the employer is obligated to make up the difference, he scoffed. That can't be true, he said. (It is true.)