A stiffer boat will have more stability and a superior RM than a tender boat,
I have also pointed out that the use of a stability diagram on a boat in a wave results in totally the wrong conclusion.
Keep in mind that the values in the stability diagram are the related to the bouyancy type force and the weight acting through the center of gravity. The force acting through the center of gravity always acts vertically. But the bouyancy force acts perpindicular to the water.
As long as the water is horizonal, the effective bouyancy force acts vertically and the weight vector acts vertically. In this case, the righting moment diagram is correct. However, when you place the boat on a wave, the effective bouyancy force acts at an angle equal to the wave angle. But the weight still acts vertically. This dramatically reduces the righting moment. There are additional secondary effects that further reduce the righting moment.
Furthermore, when we consider the dynamic effects, the axis of rotation is no longer at the metacenter. This has profound effects on how the boat heels. Some people want to analyze the dynamics as if the dynamic axis is at the metacenter. This is a severe error, particularly as the boat goes onto a wave. In the most extreme case, the dynamic center falls below the center of gravity.
So basically, we need to throw the stability diagram and metacenter out the window when we have dynamics or when we place the boat on waves. On the other hand that is why they call it the STATIC stability diagram.