Hi Callisto
Would echo everything Ian has said.
Another great dive is a Cenote that I think is called Angelina. There is a mass of decomposing matter at the bottom that has created a gas cloud at 35mish. As you descend it looks like you have hit a sandy bottom but you descend through it to the dark underneath as the cloud filters out the natural light. It's very eerie and quite a unique dive.
I dived with Aktun Dive Centre in Tulum to do this. They were outstanding but that was a couple of years ago so the instructor I had could have moved on. You can find them at:
www.aktundive.com
Enjoy Mexico!