Apparatus for monitoring one or more components of a watercraft propelling system, at least one of said components being composed of a mechanical seal device for a drive shaft extending through an opening in a watercraft hull, said device being intended to prevent water from leaking inside said hull through said opening, said drive shaft being rotatable about an axis of rotation coinciding with its longitudinal axis, which mechanical seal device comprises a stationary part connectable to said hull and a rotary part, coupled so as to rotate to said stationary part and integrally connected to said drive shaft, the stationary part and the rotary part being provided with a stationary annular sliding element and a rotary annular sliding element respectively, said annular sliding elements (10, 20) being provided each one with a planar contact face (100, 200) and the two contact faces (100, 200) being coplanar and in contact with each other, characterized in that it comprises a controller and a plurality of sensors (40, 41, 42, 43, 44, 45, 46, 47) for detecting operating parameters of said one or more components and for generating corresponding control signals, which sensors are configured so as to transmit the control signals to the controller, wherein sensors comprise sensors detecting the operating parameters of the mechanical seal device, said sensors comprising a wear sensor for the contact faces, which wear sensor is arranged for measuring the wear of one or both the annular elements so as to denote how much the faces have withdrawn during sliding and how much one or both the annular elements have worn out and therefore have grown thin, wherein said controller is set such to carry out an estimate of the residual time of the operating life of the mechanical seal device on the basis of the said control signals. which mechanical seal device comprises a stationary part connectable to said hull and a rotary part, coupled so as to rotate to said stationary part and integrally connected to said drive shaft, the stationary part and the rotary part being provided with a stationary annular sliding element and a rotary annular sliding element respectively, said annular sliding elements (10, 20) being provided each one with a planar contact face (100, 200) and the two contact faces (100, 200) being coplanar and in contact with each other, characterized in that it comprises a controller and a plurality of sensors (40, 41, 42, 43, 44, 45, 46, 47) for detecting operating parameters of said one or more components and for generating corresponding control signals, which sensors are configured so as to transmit the control signals to the controller, wherein sensors comprise sensors detecting the operating parameters of the mechanical seal device, said sensors comprising a wear sensor for the contact faces, which wear sensor is arranged for measuring the wear of one or both the annular elements so as to denote how much the faces have withdrawn during sliding and how much one or both the annular elements have worn out and therefore have grown thin, wherein said controller is set such to carry out an estimate of the residual time of the operating life of the mechanical seal device on the basis of the said control signals.