Are you sure?
We have had discussions on this in the past, and knowledgeable people have said that they have to be paid a base rate, and that rate is defined by law. Any tips are above that base rate. After one such heated discussion, the South Florida operator I was using changed the captain's spiel to say the DM works "mostly" for tips, or "tips are a big part of the income." The dive operator I have been using the last couple of years says about the same thing.
None of that matters. Any person acting in that role has a basic duty of care. A few years ago there was a South Florida incident in which a diver had a medical event on the surface, and the crew failed to save him because they did not use standard man overboard procedures. The captain/owner was preparing the fish tacos for the post dive snack when it happened. The coast guard came down hard on her for not being present during those operations and for failure to train the crew to respond properly to the diver in distress.