Because you are reading a maritime glossary entry we will focus on the buoyancy of things that float on water like ships and other watercraft.
Buoyancy does have its place in other situations like submarines and blimps but right now let’s focus on the surface of the water.
When ships were made of wood it was easy to understand why a ship would float. Since a piece of wood floats it is reasonable to think a ship made almost entirely made of wood will also float.
When someone suggested that a ship made of metal like iron or steel could also float well many sailors and some shipbuilders were skeptical.
The properties of buoyancy were not well understood by everyone at the time and the fear of change made people think a ship made of metal would sink. Nothing else made of metal would float so why would a ship? To be fair most items that came into contact with water at the time were not made of metal since it was more expensive than wood.
Surely a person who saw a metal bucket floating in a trough of water would see that metal could indeed float.
The way something has positive buoyancy, which means it floats in water, is because the amount of water displaced weighs more than the object. Displacement means how much water is pushed out of the way by an object. If the object has negative buoyancy the water it displaces weighs less than the object and it will sink.
Let’s use the example of a metal ball. If the ball is full of air it will float on top of the water with only a small portion under the water. If we measured and weighed the water that was displaced we would find it weighed exactly the same as the ball.
Now let’s completely fill our metal ball with water. When we try to float it we find that it sinks. Why? It sinks because the ball now weighs more than the volume of water it displaces. The air inside the ball in the first example is much lighter than the heavy ball filled with water.
Now if we cut the ball in half and stretch it out to make the shape of a boat we can place it in the water and it should float because it is filled with air.
This is how all boats and ships float. As long as the part of the boat’s hull that is underwater weighs more than what the boat and contents weigh it will float.
Naval Architects design ships with special hull shapes using a concept called Deadrise which makes a ship float better and move through the water with less power.
The water in the sea is heavier than freshwater in lakes and rivers because it contains salt. The weight of the water and the salt together make the water heavier or denser. This means that if you float a ship in seawater it will float higher out of the water than in freshwater since the seawater is heavier than freshwater.