Stable equilibrium. The forces of gravity (G) and the buoyancy (B) must be in line and the centre of gravity (G) must be below the metacentre (M). Whilst this is the state a vessel should be in, if the centre of gravity is too low, the ship will recover too quickly from a roll and the movement will be uncomfortable and more violent.
Neutral buoyancy. The centre of gravity and the metacentre coincide or nearly coincide. This would cause the ship to heel over to one side or the other and to remain at that angle of heel, ie the ship is experiencing loll.
Unstable Equilibrium. In this state the centre of gravity is above the metacentre. This is a dangerous state and if the ship should heel over too much she will capsize.