Sounds like another good use for the zip-loc bags that everyone packs for their cruises now. One of the small, snack-size would hold all your cards and money and fold up small enough to fit into one of these cases.
Maybe I will start a new thread called "Zip-loc bag uses on-board"
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days