Why don’t iron boats sink?

Answer

A solid iron object will sink in water because the density of iron is greater than that of water. The buoyancy force experienced by a submerged object is proportional to the volume submerged. The reason for this is that the pressure at the bottom of the object is greater than at the top, so the liquid tends to push the object upwards. You can increase that volume without increasing the weight by introducing air bubbles into iron. If you introduce enough air, you will eventually reach an equilibrium: the weight of the object is equal to the upward thrust it experiences.

Thus, a boat, even in iron, will float if its total weight is less than the weight of the water occupied by the boat.