If you have a moment on land before boarding your cruise, I would get cash from an ATM. The exchange rate will be the one banks use not the currency exchange folks. See if you have a credit card that does not charge a fee for foreign transactions if you think you might make any larger purchases.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014