we have credit cards, but we hate to use the. we always buy Ship board credit in the form of gift certificates. You still need to have a credit or debit card on file, but our bill is always paid before we board. It works for us.
If they work well for you that's great. As you correctly said, you still need a card on file to use them. A cash deposit would work as well. I personally don't recommend them as a main funding source for the S & S account because they have terms and conditions for use on the account. They are also not replaceable if lost or stolen. Look here.
They are sure nice to give and receive though.