In the present paper the reciprocal theorem between the ship hull vibration and the fluid vibration, that is, underwater sound is studied. Using the reciprocal theorem of velocity potential in the fluid medium, the reciprocity of sound in the fluid systems is introduced. On the other hand, using the reciprocal theorem of mechanical vibrations, the reciprocity of ship hull vibration systems is introduced. To combine with these reciprocities, the reciprocal theorem on the total vibrating systems is proposed, and the relationship between radiation and scattering underwater sound problem is clarified by the theorem. This reciprocity is well known as Haskind's relation in water wave engineering. To verify its usefullness and accuracy, some numerical examples are shown for sound scattering problem of 2-dimensional semi-submerged circular cylindrical shell, and it is numerically demonstrated that the shell deflection amplitude in sound scattering problem is proportional to the Kotchin function in sound radiation problem.