In this work, an experimental study of surface wave diffraction on a substrate immersed in water is presented. The Scholte wave was generated by bulk wave conversion at the extremity of the substrate. The sample was a plane plate of duralumin 10 mm thick and 200 mm long which had a defect (a tenon or a mortise) with square section dimensions close to the wavelength of the wave in the fluid. Only one obstacle perpendicular to the direction of propagation of the incident wave was investigated. The acoustic pressure at 300 mm around the diffraction zone was recorded. Phenomena of mode conversions were observed. They were the same as in the Scholte wave diffraction by a dihedral: the generation of the transmitted and reflected Scholte waves, and the generalized Rayleigh waves. The study was extended to the A wave diffraction on a thin plane plate. The dimensions of the square section of the defect were half of the plate thickness. In this case, the waves generated were generalized Lamb waves.