By Daniel Kline The three cruise lines have a problem, and it could soon get passed on to the passengers. Anyone who takes a cruise understands that the ticket price never quite reflects everything you pay. Your ticket covers your room and meals in the main dining room, the buffet, and usually a few other restaurants, but lots of additional ways to spend money exist. You can, of course, opt out of extras, but most people end up dipping into their wallets for everything from adult beverages to premium coffee, shore excursions, premium dining, and onboard purchases. Yes, you get a lot for the ba…