Like airlines and hotel chains, cruise lines reward travelers who stick with their brand. So if you find Carnival's party-friendly ships or Royal Caribbean's adrenaline-pumping activities are all you could ever want in a cruise, you can sign up for the loyalty program and start racking up points. While all major lines have a program, each line differs a bit when it comes to fare discounts and perks — and how quickly you earn bonuses such as cabin upgrades, priority boarding, drink packages, and free Wi-Fi. So how much does it really pay to be loyal? Read on to find out what the major lines offer.