Energy efficiency is getting more important due to energy cost and environmental effects. Pumping systems are important because they consume almost 20% of the total energy in land power installations. Comparing to the land installations mentioned above, the energy consumed by pumps in marine installations is much greater. In this study, it is shown that the energy consumed by pumping systems onboard ships is almost 50% of the total energy consumption. These pumps work at a constant speed, in other words constant mass flow. However, some of the systems are working under variable load. For example, seawater cooling system load depends on some parameters such as seawater temperature and engine load. Therefore, the consumed energy of this system can be reduced depending on the seawater temperature. A variable speed pump is one of the solutions to save the energy in this system, which will change the pump speed depending on seawater temperature. In this paper, the energy-saving potential of a central cooling water system is analysed. The saved energy and related annual cost of the pump occupied in the system are calculated for different seawater temperatures, and its environmental results are also discussed.