It seems that the main reason for the over-tilting of the MV Sewol, was the sharp turning of the ferry. At the same time, the ferry was overly-loaded, and the cargo shifted to one side of the ferry, changing the position of the overall centre of mass significantly. This causes the ferry to go from stable equilibrium to unstable equilibrium condition.
A restoring force is a force that restores a system to its equilibrium position. In this case, the restoring force is provided by the upthrust of the displaced water on the ferry. However, the moments created by the restoring force was not sufficient to overcome that of the over-loaded weight of the ferry.
The concept of restoring force is taught in school and our Physics tuition classes when covering the topic of Oscillations and Circular Motion. It is related to the concepts of stable, unstable and neutral equilibrium as well.