Question

why does a block of wood held under water rise to the surface when released?

Answer 1

Because the density of wood is less than the density of water

Answer 2

Because the weight of the wood is less than the weight of an equal volume of water. So when it's completely submerged, the upward buoyant force on it is greater than the downward gravitational force on it. The wood accelerates in the direction of the net force on it, exactly as one would expect.