A piece of wood is submerged deep inside a long column of water and...

When the piece of wood is released, the forces acting on it are the upward buoyant force $F_B$, the downward weight $mg$, and the downward viscous drag force $F_v$.

The net upward force is given by $F_{net} = F_B - mg - F_v$.

Initially, the velocity is zero, so the viscous drag $F_v$ is zero. Since the density of wood is less than that of water, $F_B > mg$, and the wood accelerates upwards.

As the velocity of the wood increases, the viscous drag force $F_v$ also increases.

Consequently, the net upward force $F_{net}$ decreases, which means the upward acceleration $a = \dfrac{F_{net}}{m}$ decreases continuously.

Thus, the wood moves upwards with a decreasing upward acceleration.