A wooden block, floating in water, acquires kinetic energy. According to the conservation law, energy cannot arise from nothing

A wooden block, floating in water, acquires kinetic energy. According to the conservation law, energy cannot arise from nothing, therefore, there must be a body that has given such an amount of energy. What body gives energy to a wooden bar?

When a wooden block rises in water, some of the water sinks and takes up the space that the block occupied. Sinking into the free space, the water gives the bar part of its potential energy.