I'm not sure I'm following you. We're talking about a 365 scuba destination, the business is run the whole year (with peaks and whatever, but it's not that they stops for months). The average prices to dive there is well above the common Mexican/Cozumel salary and the DM's/Guides are hired proportionally to the size of the shop.
If with the average price you pay in Cozumel the shop is not paying adequately the DM's/Guides, then there's definitively something wrong (the shop owner is a greedy b*) and it deserves to close his activity.
By tipping you're only boosting this bad behavior: you're giving money to the greedy owner that will continue to do his affairs, while the DM's/Guide have to rely on a uncertain income based on the customers kindness.
It doesn't mean that you should never tip.. it only means that a tip should never been seen as an additional amount at the top of what you have already paid. This whole $10/day is IMO stupid and dangerous (for the reasons I've explained above). Tips should be given for the extraordinary not for the ordinary.