A floating hull displaces water. The surrounding water exerts pressure on the hull, and the combined upward force balances the vessel’s weight at a particular waterline. Add cargo and the vessel settles lower, displacing more water until the forces balance again. This is why a ship can be heavy without sinking.
Why can steel float?
The steel itself is dense. The ship is not a solid block of steel, though; it is a volume that includes air, machinery, cargo, and enclosed spaces. Its average density can be lower than the water it displaces. A hull’s shape turns material into a vessel that occupies enough water to support its total weight.
What does stability add?
Floating is not the same as staying upright. The arrangement of weight, buoyancy, and hull shape determines whether a vessel rights itself after a small roll or continues tipping. Designers distribute mass and volume so ordinary waves produce recoverable motion. Cargo loading is therefore a stability problem as well as a weight problem.
Why does the route matter?
A vessel designed for an ocean crossing has different constraints from a barge on a shallow river. Water density, channel depth, wave height, bridges, and loading equipment shape the hull and its allowable draft. Buoyancy supplies the common principle, but the route determines which waterline and stability margin are useful.
Designers also model what happens when a compartment takes on water or cargo shifts. Watertight divisions and loading procedures give the crew time and reserve buoyancy; they do not make a damaged vessel invulnerable. The floating principle sits inside a larger system of margins and procedures.
The waterline is an operating limit, not a decoration. Ships mark how deeply they may sit under different conditions, because salt water, fresh water, temperature, cargo, and weather alter the margin. A vessel that floats in principle can still be unsafe if it is loaded beyond the conditions its design assumes.
The long tail is in the environments: a shallow river barge, a container ship, and a fast ferry need different compromises. Buoyancy gives them a common principle; depth, waves, cargo, and route turn that principle into different machines.