Question

why does the block of plastic released under water comes up to the surface of water

Answer 1

The rules of Buoyancy say that if the object floating in water displaces the same amount of water as the object weighs it will float. Since the block of plastic doesn't weigh very much it easily displaces enough water to float.

Answer 2

A block of plastic released in water comes to surface of water because density of plastic is less then water. Objects with lesser density will float on it, with more density will sink and with equal density will be half floating.