What is a Ship ?

Thousands of years ago people fashioned the first small rafts and dugout canoes harnessing the principle of floatation to travel and move goods.
Today, cruise ships with casinos, restaurants, and swimming pools carry millions of pounds through the world's ocean by the same principle.
Lightweight materials are used where possible.
Even so, packed tightly together those materials would surely sink.
Instead, they're shaped to take up a great deal of space and enclose a lot of air. This a giant cruise ship displaces a volume of water equal to its own mass, which lets it hover at the water's surface.
In 1912, this balance was tragically disrupted for the 900-foot luxury cruise ship Titanic. An iceberg punched holes in several watertight compartments, which flooded. Dense water replaced air, and the ship went down.
As the dense water replaced the air, the ship lost its buoyancy and eventually sank to the bottom of the sea.