An electric power generator for marine vessels includes a containment enclosure, an internal combustion engine housed in the containment enclosure and including a drive shaft, rotating at a variable rotation speed, an alternator housed in the containment enclosure and configured to receive mechanical power from the internal combustion engine and to convert it into electric power, a second cooling circuit configured to cause sea water to circulate, and an electrical power converter connected to the alternator to receive an input current, having an input frequency, and to convert it into an output current, having an output frequency. The second cooling circuit includes a first heat exchanger configured to allow heat exchange between the sea water and the electrical power converter.