I finally was able to check in with Carnival. They advised me that as long as they are "Fun Ship" Dollars AND they specifically have written both "purchase a cruise" and "On Board Credit, Apply Credit to" options on the certificate, one is able to bring them on board, go to the Purser's Desk, and have them applied as on board credit (applied to your S&S card). She did advise there are two other types of certificates out there - one being a school fundraiser type certificate - and that you are not able to use those on board.