Hint: magic has nothing to do with it.
Not everything that's heavy sinks right to the bottom of the ocean floor, and there's no greater proof than Royal Caribbean's Harmony of the Seas—which happens to be the biggest cruise ship on Earth.
The world's largest cruise ship boasts an ice-skating rink, a movie theater, 10-story dry slide, and weighs no less than 227,000 tons. And that's without any of its 6,780 passengers.
We know that giant ocean liners don't sink, even with their rock climbing walls and nightclubs. But have you ever wondered why?
It's not magic or flying unicorns that keep the Harmony of the Seas plying the open waters. It's all about buoyancy: the power of a liquid to keep things afloat.
Colossal vessels stay above water by displacing an amount of water equal to their mass (the wide, U-shaped hull helps with this). As the ship moves forward and pushes water away, the water is ceaselessly trying to return to fill the space, with an energy that forces the ship upward.
And it's not just about total weight. A solid bar of steel dropped from a cruise ship balcony will undoubtedly sink until it reaches the bottom of the sea. But a boat actually has a lot of open space. No matter how many restaurants, bars, swimming pools, and casinos they cram into these floating cities, there's still an awful lot of empty volume.
Engineers are careful to keep the average density of a ship (considering both the physical weight of the vessel as well as all the air) less than the average density of the water. After all, the ocean is massive—and extremely dense. To the seas, a cruise ship is nothing more than a leaf drifting effortlessly along the surface.
So don't spend any time stressing about this one. Your ship is an engineering (and entertainment) marvel. Just sit back and enjoy the cruise.