Nowadays Stereo-PIV (SPIV) technique has reached a level of accuracy and precision suitable to a wide use in industrial facilities. Nonetheless, performing Stereo-PIV measurements in large hydrodynamic facilities for naval applications - e.g. large cavitation tunnels, towing tanks, maneuverability basins - still poses challenging operative, technical and theoretical problems; in these regards, solutions are required to assure good quality results when dealing with deep underwater conditions, reflections from the hull model, bubbly and two-phase flows, air-water interface and air bubbles entrapment by breaking waves, to name a few. In the present work, an underwater modular probe is introduced, designed to host all the electronic and optic hardware required to perform 3C-2D velocity measurements around surface and submersible models. In particular, an application to a test case is presented; a catamaran model advancing in steady drift at different angles and velocities is investigated to study the dynamics of the keel vortices generated along the demi-hulls and their interaction with the free surface.