going up and down the hill you will notice the pitch, isn't it?

A ship is doing the same thing, although more than half of the rigid body is immersed in the water. The local buoyancy of disturbance does not add too much to the overall up and down of the waves, just like the car hits the small bumps over a hill causing vibrations, it does not change the characteristic pitch movement following the shape of the hill. The dominant frequency is the same as the wave, and the bumps are the noise only.