here are some comments I posted in another thread a few days ago..
here is the low down on Cancun.
There is one HUGE dive store and a handful of little ones. I work for the big one, which is a slick water sports marina with large custom dive boats, multiple departures, a stack of other activities to do etc, etc. The main criticisms clients have of it is that it lacks some of the personal touch due to its size, (we do have over three quarters of a million customers per year), and that our schedule is less flexible than the smaller dive stores.. It is ideal for the newer diver that wants help with their equipment, help in and out of the water and a safe guided dive and photos and videos available on every dive boat etc.
The smaller dive shops, have much smaller and older boats, variable quality equipment, less departure times and options, but pride themselves on their ability to provide a more personalized flexible service. These stores are better for those who hate having their equipment set up for them, who want to be on a boat that doesn't have other groups on them, and wants more control over their own dives.
Hope that helps a little and that I was as impartial as possible. Whatever you do, I suggest that you shop around when you get here, dont buy you course from a travel agent who will tell you anything they think you want to hear, go check out the different places yourself. MEETTHE INSTRUCTORS, that you are going to do the course with and make sure you feel comfortable with them.