Summary: Wave-structure interaction systems are examined in this paper. These systems are characterized by a nonlinear restoring force and a coupled wave-structure drag and inertial exciting force. Stability analyses of system response define domains of primary and secondary resonances and reveal the existence of nonlinear solutions. Local and global bifurcations identify possible routes to chaotic motion and their controlling parameters. The analysis shows that period doubling and tangent bifurcations are enhanced by parametric excitation induced by the wave-structure coupling. Thus, complex dynamic recently uncovered numerically are obtained semianalytically allowing the identification of instabilities and their generating mechanisms.