Device for detecting obstacles for a watercraft intended for surface navigation with relative motion direction with respect to the water and of the type provided with a longitudinal extension axis and with a load-bearing structure comprising at least one hull; said device for detecting obstacles being provided with a main extension axis (X') and comprising: - at least one housing body delimiting at least one internal chamber and provided with an emission mouth communicating with said internal chamber; - at least one active sonar sensor, housed in the internal chamber of said housing body and configured for emitting an acoustic signal beam through the emission mouth of said housing body; - support means configured for supporting the housing body of said device for detecting obstacles on the load-bearing structure of a watercraft below the water surface; wherein said housing body is configured for being fixed by means of said support means to the load-bearing structure of a watercraft with the emission mouth oriented substantially horizontally and with the acoustic signal beam of said sonar sensor oriented substantially in the direction of the relative motion of the watercraft with respect to the water in order to intercept obstacles in the advancing bulk of the watercraft; characterised in that said support means comprising at least one first hinge for allowing the free rotation of said housing body around an axis (Y') parallel to the pitch axis of said watercraft; the housing body of said device for detecting obstacles comprising a rudder finning, which is configured to orient via hydrodynamic drag and with rotation at least around the axis (Y') of said first hinge said housing body substantially in the relative motion direction of the watercraft with respect to the water; wherein said support means comprise at least one second hinge for allowing the free rotation of said housing body around an axis (Z') parallel to the yaw axis of said watercraft; the rudder finning of said housing body being configured to orient via hydrodynamic drag and with rotation around the yaw axis said housing body substantially in the relative motion direction of the watercraft with respect to the water. said device for detecting obstacles being provided with a main extension axis (X') and comprising: - at least one housing body delimiting at least one internal chamber and provided with an emission mouth communicating with said internal chamber; - at least one active sonar sensor, housed in the internal chamber of said housing body and configured for emitting an acoustic signal beam through the emission mouth of said housing body; - support means configured for supporting the housing body of said device for detecting obstacles on the load-bearing structure of a watercraft below the water surface; - at least one housing body delimiting at least one internal chamber and provided with an emission mouth communicating with said internal chamber; - at least one active sonar sensor, housed in the internal chamber of said housing body and configured for emitting an acoustic signal beam through the emission mouth of said housing body; - support means configured for supporting the housing body of said device for detecting obstacles on the load-bearing structure of a watercraft below the water surface; wherein said housing body is configured for being fixed by means of said support means to the load-bearing structure of a watercraft with the emission mouth oriented substantially horizontally and with the acoustic signal beam of said sonar sensor oriented substantially in the direction of the relative motion of the watercraft with respect to the water in order to intercept obstacles in the advancing bulk of the watercraft; characterised in that said support means comprising at least one first hinge for allowing the free rotation of said housing body around an axis (Y') parallel to the pitch axis of said watercraft; the housing body of said device for detecting obstacles comprising a rudder finning, which is configured to orient via hydrodynamic drag and with rotation at least around the axis (Y') of said first hinge said housing body substantially in the relative motion direction of the watercraft with respect to the water; wherein said support means comprise at least one second hinge for allowing the free rotation of said housing body around an axis (Z') parallel to the yaw axis of said watercraft; the rudder finning of said housing body being configured to orient via hydrodynamic drag and with rotation around the yaw axis said housing body substantially in the relative motion direction of the watercraft with respect to the water.