There are several good comments here. I would concur that you should think twice, or maybe three or four times, before trying a cenote tour if you are at all claustrophobic. I have only done the Dos Ojos system, and there are definitely some tight spots on both lines. I understand some of the other cenotes are more open. You may be able to discuss this with the dive op and find a good place to try it. I had a good experience with Yucatek out of Playa del Carmen and would not hesitate to recommend them.