Yes, Carnival has started to offer the cave tubing again. It was on the list when we were on the Legend last month. And they had a good size group sign up for it.
The only problem I see in doing an excursion that involves a good drive from Belize City is that there is only one highway going either North, South or West fromt the city so if there were to be an accident causing a road blockage you may not make it back to the ship on time so be sure to have whatever is needed if you could not make the last tender to the ship.
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, Carnival Legend Nov 2013, MSC DIVINA Jan 2014