I had wondered about your handle trimix, now I know I was right. You must be a tech diver. I was surprised how few divers I met on the cruise.
As a certified rec/tech (NAUI/PADI) & commercial mixed gas diver I understand your concerns. This is part of the cenotes systems that you can dive, just Not on this excursion! Part of the tour will take you to the anchor of the cave line where one of the mapped dive routes start.
This snorkel excursion is VERY family friendly and very safe. Consider it Disneyfied snorkeling, surface only. The guides were great, the routes were marked with floats (you can see some in my pics), you will always be able to see the exits, you are required to be in a full life jacket no matter your age or knowledge, and the gear was in great shape.
I was very comfortable taking my wife and two kids and I know the dangers that ill informed people can get themselves into in these caverns.