Please read the following. A cube submerged in water exerts a force on the water. The cube exerts a force equal to its weight on the water below it, and by Newton's third law it should feel a reaction force to this weight. This means that there are four forces on it: one due to the pressure and weight of the water on top, one due to the pressure of the water below, one due to gravity and one due to the reaction force to its weight.
But the pressure from below is due to the weight of the water above it pushing downwards, but there is no water because it was displaced, so the pressure from below can only be due to random collisions. The pressure from above is due to random collisions and the weight of all the water. The displaced water also moves up, so its weight adds to the pressure on the upper surface of the cube.
The weight of the object is balanced by the reaction force of the water below, like when you place something on a table and it does not fall. So the sum of all the forces on the object is simply the weight of the water above, plus the weight of the water displaced. Therefore no object can ever sink because the net force is downwards.
What is wrong with my line of thought? I can see that it is obviously wrong but I have no idea why.