This work presents an original numerical model for a free surface flow interacting with a spring-block system. The formulation is based on the fictitious domain approach and a penalty method on viscosity to describe the rigid solid motion. The incompressible Navier-Stokes equations are solved in the whole domain and the free surface and the body contour are captured using a Volume of Fluid method. To describe the rigid body motion, a single degree of freedom model, able to represent translation or rotation, is embedded in the code. The discrete equations are written in a well known finite volume framework over Cartesian grids. In such a context, the external spring and damping forces are represented as body forces in the solid region. The proposed strategy is tested in a sloshing damping system. The numerical results are compared with experimental data obtained within the present study. Finally, the method is used to simulate a wave energy converter system as an illustration of a rotational case.