In 2019, 30 million passengers are expected to take a cruise, according to the Cruise Lines International Association. To compete for customers, cruise lines are spending billions to outdo each other with outrageous amenities and entertainment, from a high-tech planetarium at sea to an on-board race track. Royal Caribbean alone is spending $200 million on a private island, CocoCay, in the Bahamas for guests, according to Bloomberg, and had a $115 million transformation on Navigator of the Seas ship, which has the longest waterslide at sea at 800 feet.

Now one of the latest over-the-top cruise is adventures is the first on-board roller coaster, which will be on Carnival Cruise Line's new Mardi Gras ship, and passengers will be able to ride it next year.