If you pre-pay, then you pay them as part of your cruise fare. Then it's done and you don't have to think about it. Most lines, including Carnival, allow you to pre-pay. If you don't pre-pay, then most lines will automatically bill your shipboard account for the gratuities. (you will also see this referred to as auto-tips on many cruise forums). Some people like to tip in cash the old fashioned way because they feel it's more personal. Honestly, from crew members I have talked to, and in online articles I have read, they don't care, as long as they are getting paid. They are just as happy with you thanking them as you leave the ship, so long as you have tipped in one of the methods.