When you use any card Carnival will put a hold down - as Mike mentioned - above and then may put through incremental holds throughout the cruise and then put through a final transaction for the S&S balance and release the other holds. With a debit card this can severely restrict the amount of funds available to you.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013