The reason things don't sink is all to do with density . A wooden raft floats on water because wood is less dense than water.
Density is mass per unit of volume.
A 'heavy' substance should really be described one which is of high density. A 'light' substance should be described as having low density. They have very strong forces of attraction between them. In a solid which is less dense, the particles are further apart because the forces of attraction between them are weaker. A ship made of metal does not sink because the metal hull of the ship contains a lot of air.