By Daniel Kline The two cruise line rivals battle for family customers and operate in most of the same markets. Carnival Cruise Line (CCL) – Get Carnival Corporation Report and Royal Caribbean Cruise Line (RCL) – Get Royal Caribbean Group Report battle for the same customers. Both cruise lines want families looking for a good value on a vacation and they attempt to win them over by using marketing that emphasizes how fun its cruise ships are. Royal Caribbean leans in heavily on promoting its Coco Cay private island, which has a massive pool and an added-fee water park. Carnival has built its m…