If I remember correctly, MSC only shows some of the excursions on the website for most of the ports. They usually show the most requested ones.
We are booked for a Dec. 2012 cruise on the Poseia and I just was looking and at this time I was unable to see any excursions set.
Akumal Mexico Reviews
Star Princess 2005, Sun Princess 2005
Caribbean Princess 2006, MSC Lirica 2006 , NCL Pearl 2007, Majesty of the Seas 2008, Carnival Destiny 2008, MSC Lirica 2009, Carnival Valor 2009,Carnival Legend 2010, Carnival Liberty 2010,Carnival Fantsay 2011,Carnival Valor Feb 2012, MSC Poesia Dec 2012, MSC Poesia April 2013