Buoyancy

Buoyancy is the upward force a fluid exerts on an object immersed in it, equal in magnitude to the weight of the fluid the object displaces — Archimedes' principle. A steel ship floats not because steel is light but because its hull shape displaces a volume of water whose weight exceeds the ship's own; the same steel reshaped into a solid block displaces far less water for the same mass and sinks. Buoyancy is about displaced volume, not material.

Whether an object floats or sinks comes down to comparing its average density to the fluid's: less dense than the fluid, it floats with part of its volume above the surface; denser, it sinks; exactly matched, it hovers in neutral buoyancy anywhere in the fluid, which is the operating principle behind a submarine's ballast tanks and a scuba diver's buoyancy compensator.

A floating object's stability is a separate question from whether it floats at all, governed by the metacenter — the point stability calculations pivot around as a hull tips. A ship with too high a center of mass relative to its metacenter can float perfectly well while being dangerously prone to capsizing, which is why cargo loading plans care as much about where weight sits as how much there is.

Buoyancy is not exclusive to liquids: air is a fluid too, and a hot air balloon works by heating the air inside its envelope until that air's lower density makes the whole balloon's average density less than the surrounding cooler air, generating enough buoyant force to lift the balloon, basket, and payload. The same principle, applied at atmospheric scale, is why helium balloons rise and why an object weighed in air reads slightly lighter than it would in vacuum — the surrounding air buoys it up.