By Daniel Kline The two big cruise lines compete for the same customer base. Royal Caribbean (RCL) – Get Royal Caribbean Group Report and Carnival Cruise Lines (CCL) – Get Carnival Corporation Report endlessly attempt to one-up each other. If one cruise line adds indoor skydiving, the other one installs a roller coaster. We’ve seen aqua theaters, laser tag, skating rinks, bumper cars, and an ongoing attempt to squeeze bigger and better water slides onto a cruise ship. It’s an arms race that makes sense because Royal Caribbean and Carnival both cater to families. If a better water slide, a Broa…