ADC signed a deal with Castro to split the profits (50% Fidel, 10% Cuban partner, 40% ADC) from the recovery of any Spanish Galleons. That is what ADC is really after. There are a dozen or so known wrecks in Cuban waters which have not been explored because of the depth. ADC thinks they can get down to them.
The "sunken city" is a nice cover. National Geographic can foot part of the bill without being tainted by helping to further enrich Fidel Castro.
I think National Geographic bailed out of the deal with ADC.