Experiments have been carried out to investigate the hydrodynamic interaction between a train of regular incident waves and a partially immersed, substantially rectangular obstacle in an essentially two-dimensional domain. The interaction is considered for both fixed and floating obstacle modes. Experimental measurements have been taken of the obstacle reflection and transmission characteristics in both modes, together with the obstacle motion response in the floating mode, and comparison is made with the predictions of linear diffraction theory. The results of the investigation have enabled identification of the conditions under which the use of potential theory alone becomes invalid due to the significant presence of energy dissipation mechanisms in the interactive process.