Definitely not a problem at Dos Ojos. Most cenotes are private property and could have their own rules. I was a brand-new diver (as in, it was my third dive post-OW cert); they not only let me in (after a mini checkout dive in the open water entry area); they let me bring my GoPro after I promised to focus on bouyancy first and foremost. I kept my word and only had to put a finger down at one point to stop my forward momentum, but my pictures were totally unusable. Another guy in our group, an OW instructor with cavern cert and a much better camera, got some amazing shots, so it was definitely just me.