The plasticine has a mass of 100 grams and an actual volume of 50 cm^{3}.

As a compact lump, it can only displace a maximum of 50 cm^{3} of water, which is 50 grams of water (density of water is 1 g cm^{-3}). Weight of displaced water is less than weight of the plasticine. As a solid lump, plasticine is doomed to sink in water.

When it is molded into a boat, it tries to pretend to be bigger than it actually is, volume wise at least. So how much water does it displace now? 100 cm^{3}, or 100 grams of water. Just enough so that weight of displaced water matches weight of the plasticine. That’s how boats float.