Diving is nothing like Cozumel.
Dive ops are not the quality that is possible to find on Cozumel, to put it bluntly, there is a lot of bull sh*tting going on between dive ops on the Isla, who is actually a Padi certified dive op, who just uses the Padi logo on their signage, it's up for discussion and a wink-wink, don't ask, don't tell policy is definely in effect there. Ops using single engine boats, no radios on board, reliance of cell phones for comminications, ops without oxygen on board... on and on... very shoe string operations, you'll find either a lot of seat of your pants or extremely dumbed down diving going on. Beware, you will find ops there who will take you out no matter what the conditions are, no matter what your experience level is, green backs in cash = lets go! There will be no discussion of your dive experience no asking for your C-card. The answer to any of your questions will be the standard "no problem" and a smile. So be aware and keep in mind you are a certified diver and are responsible for your own safety.
Dive sites tend to be mostly shallow 25-65 ft for the most part,(which is probably what keeps most of these ops out of trouble). Some of the dive ops are blatantly running on tight schedules, they want to be out and back according to their schedules to maximize the ROI. The shallow dives or avoiding of deeper means very short surface intervals, usually whatever it takes to leave dive number one and motor 10 minutes to dive number two. Every dive I've been on is a dive, which again is more emphasis on the schedule then on the fun of diving. Set a record diving in Mujeres with my shortest dive ever - 25 minutes. We were never allowed to dive our tank or computer, infact just leave your computer at home, you'll be better off with a stop watch.
I experienced lots of current, lots of surge, another difference between there and Cozumel - because some of the dive sites the reef area is so small, sometimes you'll find ourself going around them in a circle, meaning one side you're going with the current, then the other side you're swiming against current. Not like the long linear drifts in Cozumel. Visibility was variable from really poor 10-15 ft to average of 40-50ft depending on the site and the weather.
Diving there will be what you can make of it and vary from dive to dive based on the dive location. To be honest, most of the better dives are closer to Cancun then the Isla, and some dive ops are reluctant to go to them because of the 'schedule', gas, etc... Expect to see dives sites Machones I and II more often then you'll like.
Rental equipment is hit and miss. I had leaks, was streaming bubbles on every dive, the dive ops aren't really too concerned with stuff like that, when you're coming up with 1200-1500 psi whats a little leak here and there? We experienced two dive boat break downs over the course of 4 days. One leaving everyone drifting for about an hour before a tow arrived, not a big deal as we were on vacation and had no schedule to keep, they rented a replacement boat for the next day, it lasted 1 day until it too had problems and had to be replaced with another rental boat. Keep in mind, it's a small island, even though it is adjacent to Cancun. It's not a sophisticated dive destination like Cozumel, it operates like a small isolated island with limited resources and is still very much a fishing village island.
I've dove with both Enrique's unique dives and Aqua Adverntures.