The first person who noticed this was the Greek mathematician Archimedes, in the third century BC. According to the popular legend, he had the job of finding out if a crown made for a king was solid gold or a cheap fake made from a mixture of gold and silver. One version of the story says that he was taking a bath and noticed how the water level rose as he immersed his body. He realized that if he dropped a gold crown into a bath, it would push out or "displace" its own volume of water over the side, effectively giving him an easy way to measure the volume of a very complex object. By weighing the crown, he could then easily work out its density (its mass divided by its volume) and compare it with that of gold. If the density was lower than that of gold, the crown was clearly a fake.
Later, he came up with the famous law of physics now known as Archimedes'Principle: when something is resting in or on water, it feels an upward (buoyant) force equal to the weight of the water that it pushes aside (or displaces). If an object is completely submerged, this buoyant force, pushing upwards, effectively reduces its weight: it seems to weigh less when it's underwater than it does if it were on dry land. That's why something like a rubber diving brick (one of those bricks you train with in a swimming pool) feels lighter when you pick it up from underwater than when you bring it to the surface and lift it through the air: underwater, you're getting a helping hand from the buoyant force.
In conclusion, a huge boat will float because, even though it weighs a lot, it displaces an enormous amount of water that weighs even more. Furthermore, boats are designed specifically so that they will displace enough water to assure that they’ll float easily.
Written by Paolo Abate and Martina Borrelli