Most cruiselines allow cancellation without penalty as long as it is done at least 24 hours prior to the excursion. I have not had experience with this particular line though.
Mary Lou Scanlon
NCL Pride of America April 24, 2010
NCL Epic February 12, 2011
RCCL Allure of the Seas - September 18, 2011
Celebrity Eclipse - February 11, 2012:
RCCL Navigator OTS - February 9, 2013