We have three identical buckets filled with water to the brim. In one of them we introduce a piece of wood of half a kilo that dislodges a part of water and is floating and in another we introduce a plastic ball that weighs a quarter of a kilo that also dislodges some water and is floating.
If we weigh them on a scale, Which cube will weigh more? The one that has only water, the one that has the piece of wood of half a kilo floating or the cube in which we have introduced the ball of a quarter of a kilo.
The three cubes weigh the same.
It is true that in some buckets there is less water than in others, since the introduced objects that float dislodge a certain volume of it. But according to the principle of flotation, any floating body dislodges, with its submerged part, an amount (by weight) of liquid exactly equal to the total weight of the object so that the three cubes will weigh exactly the same.