My family and I will be doing the Disney Excursion to Xcaret on the 5/11 inaugural. I will find out the answers to your questions about whether Disney will let people off of the ship who are not booked on the excursion. Quite frankly though, it is not much cheaper to do the Disney excursion than it would be to go yourself. Admission to the park is, according to the Xcaret website, is a minimum of 75.00 and that does not include the ferry over to the mainland (if Disney will not let you off with the others), or food like the Disney tour does. 99.00 is really not all that bad for a 7 hour trip...Just my thoughts but I will let everyone know by offering a complete trip report...
Jennifer