Question

Steal has a density much greater than water, why do massive cargo ships still float?

Answer 1

the Ship's hull is a container of material (air mostly) that is lighter than an equal volume of water.

When an Iron ship that weighs 100,000 tons floats in the water,

the weight of the water it displaces is exactly the same.

But the volume of 100,000 tons of water is much less than the volume of the ship's hull in the water.

In other words the ship is taking up more space in the water than the same tonnage of water takes.

The ship (including the space inside) is less dense than the water it displaces, so it floats.

If you fill up the space inside the ship with water, the ship will sink because the stuff the ship is made of (iron) is denser than water, and once the inside of the ship has enough water in it,

then the ship full of water is more dense than the water it displaces and it sinks.