Presently, as a result of the technical progress, more and more complicated machines are being used in our everyday life. This is even more the case in relation to technology used at sea, where highly specialised services are needed. Sophisticated marine devices require special diagnostic methods that take into account the specific conditions of use of this type of machinery. In this paper we present the diagnostic systems elaborated to support the exploitation of the vessel power. Marine engine is a complex technical object. For the purpose of diagnostics it is convenient to divide the engine into several units - subsystems such as: piston -crank assembly; working medium exchange system, fuel supply system, lubricating system, cooling system, starting up - reversing system; combustion chamber, etc. The organization of the marine engine and auxiliary machinery diagnostic process can usually come down to two stages, general diagnostics and damage location. Most popular in marine engine diagnostics have been the periodic run analyzers, called pressure analyzers, electronic indicators. The diagnostic system of marine engine is able to assess the current engine condition and give forecast concerning its future operation in a complex way with the use of computer technology. Working out operating decision was based on proper preparation of operational parameters which were processed in a computer according to defined algorithms.