This is an excerpt from Steve Gerrard's website about his dive down to 150 feet in 7 Bocas Cenote.  The link is http://www.deephorizon.info/index.php

I decide at this point this is a good tie-off place because there is a rare formation securing the end of the horizontal line. So far, all the walls of the cenote are very smooth. I tie-off my guideline and begin my descent. I pass the 96 feet/29 meters level back into the saltwater and hydrogen sulfide. My face begins to sting, however that is always normal me for when diving saltwater, though, I must admit this was a little bit more intense. Descending the wall, carefully watching my gauge and my guideline I pass the 130 feet/39.4 meter. Wow, this cenote is going deeper than expected. The visibility is only about 15 feet/4.5 meters. I notice my lips beginning to feel numb. I decide to descend further because I really wanted to confirm the depth and hoping this was simply a hydrogen sulfide layer as like in Cenote Angelita 10 kilometers south of Tulum. At 155 feet/50 meters, no bottom, no improvement in visibility and no clear saltwater zone.

My lips were totally numb, my face still acutely stinging, the facial muscles around my eyes were aching and I was now feeling nauseated. My eyes felt as if they wanted to shut down. I was in trouble. I remembered an email story written by Matt Matthes of Playa Del Carmen several years ago where he and a friend were diving a deep cenote in the State of Yucatan (Merida) near the Gulf of Mexico. They descended into the cenote and came upon hydrogen sulfide of an intense level. After 10 minutes into the dive they were experiencing the same symptoms as I was now. They immediately ascended and got out of water. That is exactly what I did as I concentrated on reeling my guideline and ascending. I was really praying that my symptoms would not get worse. I got back into the fresh water and the stinging sensation began to reside. However, my lips and facial muscles were very numb and I still felt queasy. I reached 15 feet/4.5 meters level for my safety stop and all my symptoms were withdrawing. At the surface with a total time of the dive at 22 minutes I distinctly knew I got myself into a highly concentrated area and dangerous level of hydrogen sulfide. I floated at the surface for about 10 minutes angry that I had put myself into a dangerous situation not because of the depth; because I should have known better to back off once I felt that stinging sensation on my face. That was a clear signal of the high concentration of H2S. You learn by experience as this was my first time encountering this kind of environmental hazard. This cenote will be designated a dangerous dive site unless diving 90 feet/27 meters or less. In addition, I will return within the next few weeks with Sergio Granucci and from a depth of 90 feet/27 meters we will plumb for the bottom using a fiberglass tape with a lead weight and utilize my ScubaPro sonar gun to try to hopefully determine the actual depth of this unique cenote.