You haven't addressed the economic realities. Competition pressures the small-business dive shops to lower per-tank prices (including competition from non-SCUBA activities!). Paying DMs more pressures the small-business dive shops to raise per-tank prices. There's a balancing act between those two. Having low-morale, less capable or caring DMs repulses customers while high-morale, more capable and caring DMs attracts more customers. There's a balancing act there, too. The small-business owner has to balance all of those things or go out of business.
Higher per-tank prices pressure customers to lower-priced competitors, meaning the business can't afford to pay DMs, especially when they go out of business. Relying on tips allows shops to keep lower per-tank costs (to attract customers) while still compensating DMs through tipping. DMs will naturally gravitate to where the shops pay them more and treat them better. If you think that you can open a dive shop there that can keep prices low and stay in business while paying DMs more of your revenue, then sounds like a great business opportunity for you. With low prices and happy DMs, I'm sure you'll be very successful. However, I bet others have thought the same thing.
Tipping is the mechanism to compensate DMs in a very competitive market that requires lower up-front costs to stay in business while rewarding them for taking care of you. The better they treat you, the customer, then the better you compensate them, and they are incentivized to continue treating customers well. I'm willing to bet money that none of the Cozumel dive shop owners are 1% fat cats just rolling in the money they are stealing from their employees. All it would take is one 'good' shop owner to pay themself less to keep prices low and treat the employees right, and all the workers would flock to that shop. That could be you!