I have been diving Cozumel for nearly 20 years, and IME, unless folks are being more stealthy than I am about it when I slip the DM a tip, more people do not tip at all than tip at any rate. I'm not saying it's right, but that's what I have seen.I always use Dollars to pay for diving and tipping the divemasters. But I also almost always get reservations ahead of time for a 4 or 5 day dive package, which usually requires reservations and a deposit up front. If I happen to overtip DMs by tipping in Dollars, I usually give $15-$20 a day for a two-tank dive, that's one cost I am more than happy to live with. Too many DMs do a great job and get stiffed by Divers who are still living in the 1980s and think $5 for a two-tank dive is plenty.