From the video, can you figure out the density of the plasticine used?
The plasticine boat has a mass of 100 grams (see 0:05)
When floating as a boat, it displaces 100 cm3 (see 0:21) which corresponds to 100 grams of water. Just enough so that weight of displaced water matches weight of the plasticine.
As a compact lump, it displaces 50 cm3 of water (see 0:33), which corresponds to 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 sinks in water.
Note that the actual volume of the plasticine is 50 cm3. But when moulded into a boat, it can displace more than its volume of water. That’s how metal ships float.
The density of plasticine is M ÷ V = 100 g ÷ 50 cm3 = 2 g cm-3.