The problem with using a debit card for the sign and sail card are the holds. Carnival will put a hold on the card for the dollar amount of each purchase on the ship. In a perfect world, these holds would fall off once Carnival pushes through the actual charges, but unfortunately, thsi isn't a perfect world.
Because each hold is for a small amount (depends on the passenger of course) they don't fall off when the final charges are assessed because the final charges are in a lump sum. For instance, let's say you have:
in charges, and the final bill is $70, Carnvial pushes through a one-time charge of $70. The banks and credit card companies use automatic processing and because the amounts don't match exactly, the holds to not fall off automatically, which leaves your account on hold for twice what you spent on board.
The holds will eventually fall off, but not for 10-15 days, the legal amount of time a merchant has to send in the official paperwork proving the charge is valid.
I would recommend that if this person does not have a regular credit card, that they use cash to open their Sail and Sign card and not have to deal with the holds on the same funds.
Hope that helps!