"I guess all the Mexican divers are in the USA."
Doing the diving Americans won't do.
Seriously, American cave divers are very active in Mexico. The Yucatan is underlain by a most remarkable aquifer that explorers are systematically documenting. Some groups are using cutting edge (civilian) rebreathers and spending up to 18-hours underground and underwater. There are Mayan artifacts and ancient animal bones to find. It is all very exciting if one enjoys that kind of thing.
A question for you or anyone:
Did the impact from 65 million years ago create the conditions for these underwater caves and rivers?