I have logged in excess of 250 dives in the Cancun / Cozumel area. In Cancun the only sharks we have seen are as time mentioned a Reef Shark and several Nurse Sharks.
Both are non threatning, I have dove many times with them within 10 feet, they are bottom dwellers and posses no danger to humans unless you frighten them. Many dives we have touched them, they simply swim away from the area.

Regardless of where you go diving you will hear roumors of what dangerous marine life people have seen ect however few can ever relate exact facts to support their claims. I am not saying that something might not accidently travel into the Cancun area however it is highly unlikely.

Sharks are very beautiful and I love just watching how they move without effort. I guarentee you they are far more leary of you than you can be of them.

Enjoy your dives and enjoy everything the underwater world has to offer, it is simply a sight to behold.