Why do boats float? As usual, the indispensable Wikipedia provides a clear answer. Displacement. Old Archimedes back in the days of classic Greece gets first credit for this theory. So it’s been around for awhile.
This theory works because the average density of a boat — the combination of the steel and the air — is very light compared to the average density of water. Also worth visualizing that the boat will sink into the water until it has displaced an amount of water equal to the weight of the boat. This from howstuffworks.com. Pressure also figures in to this miracle of nature. The same link above also works through this pressure concept. I found both sites well worth reading. Now if Bernoulli’s principle governing flight were as easy to understand. The two are a bit related by the way.
Also worth noting that displacement varies by the density of water being displaced. Seawater (1025 kg/m³) is more dense than fresh water (1000 kg/m³). So boats will ride higher in salt and lower in fresh.
Just something to think about next time you belly up to the bar for your second bowl of Cap’n Crunch. Heed your displacement numbers.