The plasticine has a mass of 100 grams and an actual volume of 50 cm3.
As a compact lump, it can only displace a maximum of 50 cm3 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 cm3, or100 grams of water. Just enough so that weight of displaced water matches weight of the plasticine. That’s how boats float.
Act big. Floats better.