Floating offshore structures such as wind turbines and maritime surveillance platforms experience pitch vibrations caused by wind and wave induced forces which affect their reliability, safety, and efficiency significantly. This paper presents a novel semi-active control system for the pitch stabilization of an offshore platform. We propose a semi-active control system, by which the hydrodynamic forces raised by the motion of a fin-shaped submerged damper mass are utilized to adjust its damping ratio. In this regard first, the grey relational analysis is applied to optimally tune the mass ratio of a passive submerged structural stabilization system. The performance of the stabilization system is then studied in frequency domain for different angles of attacks of the fin damper mass. A rule-based control strategy, where the tuned passive stabilization system changes to a semi-active stabilization system is introduced. In the proposed rule-based control strategy the angle of attack of the fin damper mass is controlled by a scheduler with regards to the collected wave frequency. Based on the obtained results, while the optimally tuned passive stabilization system suppresses the pitch vibration of the offshore platform up to 48.77%, in the proposed semi-active system, the suppression rate rises up to 64.79%.