Abstract: A method for filtering seawater onboard a ship involves pumping seawater into a filtration device so that the seawater is introduced into the filtration device at an inlet pressure, flows in the filtration device through a filter element, and after the filter element as filtered seawater has an outlet pressure. A concentrate phase or concentrate removed using the cleaning device at the filter element of the filtration device is carried away from the filter element and has a concentrate pressure. The inlet pressure, the outlet pressure, and the concentrate pressure are measured. A change in filter efficiency of the filter element is recognized by determining a change in a contamination pressure difference between the inlet pressure and the outlet pressure and/or a suction pressure difference defined as the difference between the outlet pressure and the concentrate pressure is controlled depending on the contamination pressure difference.