A diagnostics system for estimating operational efficiency of a marine vessel that employs a propulsion system including a propeller mounted to a rotatable shaft for converting rotative shaft power transferred from the shaft to the propeller into thrust to propel the marine vessel across water, the diagnostics system comprising a plurality of sensors arranged to measure a respective characteristic of the marine vessel operation, said sensors comprising a thrust sensor arranged to measure the thrust generated by the propeller, a torque sensor arranged measure the torque in shaft, a rotational speed sensor arranged to measure the rotational speed of the shaft, and a speed sensor arranged to measure speed through water of the marine vessel, a data acquisition means for obtaining measurement values from said plurality of sensors (220, comprising measurement values descriptive of the shaft power, the thrust and speed through water of the marine vessel; a data analysis means for estimating, on basis of said measurement values, at least one of first excess shaft power caused by fouling of the propeller and second excess shaft power caused by fouling of the hull of the marine vessel, wherein the estimation of the first excess shaft power is carried out separately from the estimation of the second excess shaft power, the data analysis means arranged to estimate the first excess shaft power in dependence of the shaft power and thrust at a first moment of time in view of the speed through water of the marine vessel at the first moment of time and further in view of shaft power and thrust required for said speed through water in a predefined reference condition, and/or estimate the second excess shaft power in dependence of the shaft power and thrust at a second moment of time in view of the speed through water of the marine vessel at the second moment of time and further in view of thrust required for said speed through water in a predefined reference condition; and an evaluation means for issuing at least one of an indication concerning propeller cleaning at least in dependence of the first excess shaft power and an indication concerning hull cleaning at least in dependence of the second excess shaft power, the evaluation means arranged to issue the indication concerning propeller cleaning in response to the first excess shaft power or a value derived therefrom exceeding a first predefined threshold value, and/or issue the indication concerning hull cleaning in response to the second excess shaft power or a value derived therefrom exceeding a second predefined threshold value. estimate the first excess shaft power in dependence of the shaft power and thrust at a first moment of time in view of the speed through water of the marine vessel at the first moment of time and further in view of shaft power and thrust required for said speed through water in a predefined reference condition, and/or estimate the second excess shaft power in dependence of the shaft power and thrust at a second moment of time in view of the speed through water of the marine vessel at the second moment of time and further in view of thrust required for said speed through water in a predefined reference condition; and issue the indication concerning propeller cleaning in response to the first excess shaft power or a value derived therefrom exceeding a first predefined threshold value, and/or issue the indication concerning hull cleaning in response to the second excess shaft power or a value derived therefrom exceeding a second predefined threshold value.